Add support for CMake's find_package()

[claremacrae]

This was mentioned in #75 - and will provide another nice way of making ApprovalTests available to users...

[claremacrae]

This from 2016 may be useful https://foonathan.net/2016/03/cmake-install/

[1fabrism]

Hello. I'm just a beginner, but as I am learning CMake I came across this page which recommends favoring cmake config files instead of find_package:

If you are the library author, don't make a Find.cmake script! These were designed for libraries whose authors did not support CMake. Use a Config.cmake instead as listed below.

I hope this is helpful