Add suffix options for generated classes

[fangliml]

I'm integrating Wire in a very large and old codebase wich already uses protoc. It would be awesome to have the option of adding a suffix to the generated classes (not only the services), to avoid conflicts with old classes.

[oldergod]

Currently, the only thing doable to differentiate them would be to use the wire_package file option which will tell Wire to override the java_package or proto package when generating files.