Thoughts on customization

[Keboo]

So, I wrote out the code I was thinking of in the meeting.

Final thoughts:

1. This did not look as good as it did in my head.
2. I am realizing that simply having the generator put in partial methods (as you described in your README) as well as generating the classes as partials, actually gives app developers quite a bit of control. My current thinking is figuring this out is the best option.
3. I left a comment in there, that it may be nice to consider having the generator append interfaces or methods to the generated class by accepting inputs from MSBuild variables. This would give people who want to extend JackFruit even more control, and would eliminate the need for them to write Roselyn generators. A simple example is the customize method in this PR, if a library author could add a variable to include an interface on the generated classes, it would allow for extension methods on those types to be used.