Add pkg-config integration.

[jhunterkohler]

It could be good to add a pkg-config file and include it in other integrations. I honestly haven't written one before, but I think it would be something like this. Very short. Just a thought because I use Mac/Homebrew/CMake. 😁

prefix=/usr/local
exec_prefix=${prefix}
includedir=${prefix}/include
libdir=${exec_prefix}/lib

Name: termcolor
Description: Header-only C++ library for printing colored messages to the terminal.
Version: 2.0.0
Cflags: -I${includedir}/