Allow passing a file extension to the compile and clean rake tasks

Allows external gems to use these tasks by specifying the types of
files they intend on generating, such as `.h` or `.myaweomseplugin.rb`.
The default is of course `.pb.rb` as that is the extension of files
generated from this library.
Merge pull request #143 from localshred/bj/compile-file-extension-arg

Allow passing a file extension to compile/clean