Add library config options

[elihart]

This adds support for optional library wide configuration options to the annotation processor. The two options available so far are requiring abstract model classes and requiring fields with @EpoxyAttribute to implement hashCode (or use hash=false).

This is done via a new @EpoxyConfig annotation that can be used anywhere in the project. My thinking is that an empty class could be created to do this. Something like

@EpoxyConfig(...)
final class EpoxyConfig {
  private EpoxyConfig(){}
}

I haven't seen any other ways for specifying options to an annotation processor, if someone has better ideas on how to do this let me know!

@felipecsl @seanabraham @gpeal @ngsilverman