Add "process" option for tasks

Sometimes it's necessary to replace some information in coffee files before compilation.
You could use some kind of macros in your files, e.g.:
> mongoose.connect('mongodb://__my_mongo_db_url__');

After that you could create dev build for local testing by replacing '__my_mongo_db_url__' to '__localhost/SomeLocalDB__'
or create production build replacing '__my_mongo_db_url__' to '__remotehost/SomeRemoteDB__'.
Also it could be some other cases that requires some replacements in the coffee files contents.

The same option already exists in [grunt-contrib-copy#process](https://github.com/gruntjs/grunt-contrib-copy#process), but it requires installing additional package and creating one more grunt task/watch to do simple replacement operation.