Added basic filter functionality

[mjvmroz]

I've added an attribute for filtering, the reason being that the directive did not support formatting numbers. I don't think that floating points should be disabled by default, but I maintained existing behaviour so as to avoid breaking sites using a previous version. If the countFilter attribute is present, basic checks are performed, but it is assumed that the filter is registered and that the parameters are correct.

I have also updated the documentation accordingly, and updated package.json to point at the correct pages.