Made CMakeLists.txt CPM-compatible

[sdeleon28]

Implemented the fix detailed in #253

In summary this PR:

• Makes it possible to include react-juce with CPM
• Makes it possible to modify REACTJUCE_JS_LIBRARY from client code
• Makes building JUCE optional
• Makes building the example optional
• Documents all of these changes

[sdeleon28]

I'll fix that linting error in a sec

[JoshMarler]

@sdeleon28 , apologies for the delay on this one. In crunch time at current shop. I'll do my utmost to take a first pass over this tonight!

[mjmaurer]

Any updates here? Would love to have this merged :)

[nick-thompson]

I'm for it, but we had originally held off on merging this because we had come up with a different way that we wanted to structure our CMake here. In particular, I remember @JoshMarler suggesting that the root level project shouldn't add juce, that we can delegate that to whomever calls add_subdirectory(react_juce), and therefore we can move the juce inclusion here into the example project. I think that structure would clean things up a bit still, so I think I'd prefer that. Am I missing anything there @JoshMarler? Was there more to it?