Adding BUILD_MIDILIBRARY_ONLY to CMake

[Ricardicus]

Thanks for this project!

I use the midifile library, which is great, but I don't use the example programs.
Adding an option to optionally not build them.

[rettinghaus]

same as #59 and #93

[rettinghaus]

@craigsapp seems to be popular demand about this.

[craigsapp]

Thanks for reminding me @rettinghaus! This is a very good option to add. It might even be better to suppress compiling of the example programs by default and only compile the library (the tools are more for demonstrations for using the library in various situations).

[JayFoxRox]

This should probably have covered https://github.com/craigsapp/midifile/blame/98917df5b1bf0d6e8d4c0e5fff86d6b05343e793/CMakeLists.txt#L185-L203, too