Add read filter option

[jamesspencer]

Inspired by issue #2 https://github.com/cgross/grunt-dom-munger/issues/2

[ProLoser]

@jamesspencer I'm curious what the filter function you're using is.

[jamesspencer]

We needed to strip any leading slashes from script sources before passing the results directly to another 3rd party task:

{
  selector: 'script[src]',
  attribute: 'src',
  writeto: 'myJsRefs',
  isPath: true,
  filter: function(val) {
    return val.replace(/^\//, '');
  }
}

Looking back, filter is the wrong term to use as this is about modifying the data rather than determining whether it should be added to the collection.

[ProLoser]

Yeah. I would just change the property to callback. You also could improve performance by simply doing an if-set check instead of declaring a noop.

[ProLoser]

Actually. It would make sense if it worked as both a cleanup AND filter. Such as if you return 'false' it doesn't get added to the collection.

[jamesspencer]

Changing to callback sounds good, also performance note.

RE filtering: this did cross my mind but by allowing the user to manipulate the data also allows then to return non-string values; should their intent be to collect a list of booleans (based on some criteria) then this functionality would strip results. A more flexible and explicit design would have separate callback and filter operations.