Use locals as `this` value inside filter functions

[wankdanker]

This little patch changes the value of this inside all filter functions to that of the locals object. Currently it seems that this is a reference to the filters object. The purpose is to expose additional data to the filter function without having to pass additional arguments from the template.

This could potentially be a breaking change if users are currently relying on this being a reference to the filters object from within their custom filters. Additional changes could be relatively easily made to make this configurable.