Added support for template inheritance

Jade extends and blocks now work and a new option “resolvePath” has
been added. Test and fixtures and README change included.
Why exactly are you overridding the `processFile` method with the exact same code?

https://github.com/broccolijs/broccoli-filter/blob/e8ebda171377a26d10a3549911efc7a4114ff4dc/index.js#L131-L140