Adding a filter function for the copy operation

This change introduces the possibility of adding a filter function to the copy operation, with the addition of a `filter` key:
```
filter_function: {
  src: [ 'some/source/file.js'], 
  dest: 'our/destination/directory/', 
  filter: function( old_dest ){
    return 'another/path/to/put/atop/+old_dest;
  }
},
```
This means that the user is able to control the destination path in more detail, for instance when certain elements of the src path needs to be excluded, or certain files needs to be filtered from the copy operation.

This possibly also addresses https://github.com/gruntjs/grunt-contrib-copy/issues/82