Add meta-annotation support

[thmasker]

Please feel free to comment any improvement (I'm sure there are lots). I implemented the best solution I could workout.

The basic idea around this was iterating through the @Builder annotated elements. If the element is an annotation, retrieve both @Builder and @BuilderInterfaces (this one could be optional) and process them exactly the same way this library was already doing.

Fixes #14

[skinny85]

Thanks a lot for the PR @thmasker!

Before I dive into the details of the change, would you be so kind and do a few style changes that align the PR with the rest of the Jilt codebase?

• Imports - java* imports last in the order, and javax.* before java.*. If you're using Intellij, the settings for that are here:

• 4 spaces for indentation
• no final for local variables or parameters (final for fields is great when applicable)
• no empty line after the first line of a class declaration, and no empty line before the closing brace of a class declaration
• space between if / for, and the opening parenthesis

Thanks again for taking the time to submit the PR!

[skinny85]

Thanks @thmasker! I have a few more minor comments, but I've decided I'm going to address them myself - it'll be faster than going back and forth in PR comments 🙂.

Thanks again for taking the time to submit this PR!