Externals: Add libfmt 5.3.0 to externals

Allows us to migrate off of printf specifiers and have more type-safe
formatting facilities. It also allows for custom type support as well.
fmt is also on track to have part of it standardized within C++2a, so
this will also lessen the transitional work necessary later on by
allowing new code to use it.

This simply adds the library but doesn't do anything with it yet.

Externals/fmt/support/rst2md.py

@@ -0,0 +1,159 @@
+#!/usr/bin/env python
+# reStructuredText (RST) to GitHub-flavored Markdown converter
+
+import re, sys
+from docutils import core, nodes, writers
+
+
+def is_github_ref(node):
+    return re.match('https://github.com/.*/(issues|pull)/.*', node['refuri'])
+
+
+class Translator(nodes.NodeVisitor):
+    def __init__(self, document):
+        nodes.NodeVisitor.__init__(self, document)
+        self.output = ''
+        self.indent = 0
+        self.preserve_newlines = False
+
+    def write(self, text):
+        self.output += text.replace('\n', '\n' + ' ' * self.indent)
+
+    def visit_document(self, node):
+        pass
+
+    def depart_document(self, node):
+        pass
+
+    def visit_section(self, node):
+        pass
+
+    def depart_section(self, node):
+        # Skip all sections except the first one.
+        raise nodes.StopTraversal
+
+    def visit_title(self, node):
+        self.version = re.match(r'(\d+\.\d+\.\d+).*', node.children[0]).group(1)
+        raise nodes.SkipChildren
+
+    def visit_title_reference(self, node):
+        raise Exception(node)
+
+    def depart_title(self, node):
+        pass
+
+    def visit_Text(self, node):
+        if not self.preserve_newlines:
+            node = node.replace('\n', ' ')
+        self.write(node)
+
+    def depart_Text(self, node):
+        pass
+
+    def visit_bullet_list(self, node):
+        pass
+
+    def depart_bullet_list(self, node):
+        pass
+
+    def visit_list_item(self, node):
+        self.write('* ')
+        self.indent += 2
+
+    def depart_list_item(self, node):
+        self.indent -= 2
+        self.write('\n\n')
+
+    def visit_paragraph(self, node):
+        pass
+
+    def depart_paragraph(self, node):
+        pass
+
+    def visit_reference(self, node):
+        if not is_github_ref(node):
+            self.write('[')
+
+    def depart_reference(self, node):
+        if not is_github_ref(node):
+            self.write('](' + node['refuri'] + ')')
+
+    def visit_target(self, node):
+        pass
+
+    def depart_target(self, node):
+        pass
+
+    def visit_literal(self, node):
+        self.write('`')
+
+    def depart_literal(self, node):
+        self.write('`')
+
+    def visit_literal_block(self, node):
+        self.write('\n\n```')
+        if 'c++' in node['classes']:
+            self.write('c++')
+        self.write('\n')
+        self.preserve_newlines = True
+
+    def depart_literal_block(self, node):
+        self.write('\n```\n')
+        self.preserve_newlines = False
+
+    def visit_inline(self, node):
+        pass
+
+    def depart_inline(self, node):
+        pass
+
+    def visit_image(self, node):
+        self.write('![](' + node['uri'] + ')')
+
+    def depart_image(self, node):
+        pass
+
+    def write_row(self, row, widths):
+        for i, entry in enumerate(row):
+            text = entry[0][0] if len(entry) > 0 else ''
+            if i != 0:
+                self.write('|')
+            self.write('{:{}}'.format(text, widths[i]))
+        self.write('\n')
+
+    def visit_table(self, node):
+        table = node.children[0]
+        colspecs = table[:-2]
+        thead = table[-2]
+        tbody = table[-1]
+        widths = [int(cs['colwidth']) for cs in colspecs]
+        sep = '|'.join(['-' * w for w in widths]) + '\n'
+        self.write('\n\n')
+        self.write_row(thead[0], widths)
+        self.write(sep)
+        for row in tbody:
+            self.write_row(row, widths)
+        raise nodes.SkipChildren
+
+    def depart_table(self, node):
+        pass
+
+class MDWriter(writers.Writer):
+    """GitHub-flavored markdown writer"""
+
+    supported = ('md',)
+    """Formats this writer supports."""
+
+    def translate(self):
+        translator = Translator(self.document)
+        self.document.walkabout(translator)
+        self.output = (translator.output, translator.version)
+
+
+def convert(rst_path):
+    """Converts RST file to Markdown."""
+    return core.publish_file(source_path=rst_path, writer=MDWriter())
+
+
+if __name__ == '__main__':
+    convert(sys.argv[1])