Cache invalidation bug

Heya, I've been using your plugin and came across the following bug: The `getReferences` function doesn't recurse into included files, so if `a.pug` includes `b.pug`, which includes `c.pug`, `a.html` will only be rebuilt if `b.pug` changes, but not if `c.pug` changes.

Rather than fixing this particular issue, I'd prefer redoing the caching logic: Instead of having broccoli-pug try to figure out the dependencies, it might make sense to add some hooks to Pug so that instead of always compiling files, it gives broccoli-pug a chance to return an AST from cache. (I'm a bit short on time, but happy to elaborate more on this later, if you like.) I've gone ahead and opened an issue on the pug repo: https://github.com/pugjs/pug/issues/2668 Hopefully they'll have some thoughts on where to best add hooks.

Start by reading the getReferences function and reproducing the a.pug → b.pug → c.pug invalidation case described in the issue. Then review the linked Pug issue about cache hooks; done means agreeing on and implementing a viable caching or dependency approach that rebuilds a.html when c.pug changes.

Written by the indexing model from the issue text.