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/rtd/theme/layout.html

@@ -0,0 +1,17 @@
+{% extends "basic/layout.html" %}
+
+{% block extrahead %}
+<meta charset="UTF-8">
+<meta http-equiv="refresh" content="1;url=http://fmtlib.net/latest/">
+<script type="text/javascript">
+    window.location.href = "http://fmtlib.net/latest/"
+</script>
+<title>Page Redirection</title>
+{% endblock %}
+
+{% block document %}
+If you are not redirected automatically, follow the <a href='http://fmtlib.net/latest/'>link to the fmt documentation</a>.
+{% endblock %}
+
+{% block footer %}
+{% endblock %}