Implement support for Grunt-0.4's standard file mappings and options format.

Having played around with a few Grunt plugins since 0.4 it was noticeable that the method for specifying input/output paths was non-standard in grunt-peg. This PR brings full support for the standard configuration style as described at http://gruntjs.com/configuring-tasks whilst seamlessly maintaining support for the previous configuration style so as not to cause problems for existing users. Now it's possible to concatenate multiple input files into a single grammar, or use Grunt features like dynamic mapping and wildcards to build many grammars from a single target.