Adding compare function for sorting files for glob helper.

Glob helper now takes compare function as 2nd parameter. The function
will be passed two objects and is expected to return 1 if a >= b or
-1 otherwise. IF you want descending sort order, then 1 and -1 should
be reversed.

This compare function can be defined in options passed to assemble:

    assemble: {
      options: {
        ..
        compare_fn: function(a,b) {return a.index >= b.index ? 1 : -1},
        ..
      }
    }