add support for processing copy/directory file contents via function.

coming from grunt-init, i found generator to be lacking in this area as far as custom functions, init basically has a predefined process function that runs through template engine but i think there is value in supporting both template and custom so that you are only having to write out your custom vs adding in common boilerplate for template..

by allowing copy to process file contents via function, one can do non-template replaces, for example I have several plugin boilerplates that I like to test without template markup or multiple copies of source. so i use a keyword and replace it during copy process.