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/index.rst

@@ -0,0 +1,2 @@
+If you are not redirected automatically, follow the
+`link to the fmt documentation <http://fmtlib.net/latest/>`_.