Renamed unprefixed LOADER typedef.

Types declared in public headers should have a prefix.  The typedef has
been renamed to the more OpenGL-like GLADloadproc.
Renamed function pointer prefixes.

The client API function pointer typedefs have been renamed to the
standard PFN<function>PROC pattern.  An underscore has been added
between the glad prefix and the client API function name to make it
easier and quicker to tell where one ends and the other begins.
Merge pull request #3 from elmindreda/renaming

Renamed loader and client API pointer typedefs