allow adding custom supportedTypes

[unkstar]

I develop this feature because I have to put a web worker's script on CDN, and inspired by pattern option from grunt-usemin

new option work like this:

    //define worker type
    function Worker_cdn() {
    };

    Worker_cdn.prototype = {
        run : function() {
            var self = this;
            return this.buffer.replace(/new\s+Worker\(\s*['"]([^"']+)['"]/gm, function(match, url) {
                return match.replace(url, self._replace(url));
            });
        }
    };
    //and add it to supported type

    cdn: {
            options: {
                cdn:'http://example.com/static/',
                flatten:false,
                supportedTypes:{"js":Worker_cdn}
            },
            dist: {
                src: [
                    '<%= yeoman.dist %>/*.html',
                    '<%= yeoman.dist %>/scripts/*.js'
                ]
            }
       }

This patch let user inherit from Job interface directly, this give user great flexibility, or maybe too much? Job._replace may need a better name too.

[unkstar]

4dadf75 fix the same problem as #45 , I prefer #45 than my own solution, please pull #45 intead

By the way, I can't find a reasonable way to write a test on this feature in orignial tests framework, it need a fully functioning grunt to work.