Use locals as `this` value inside filter functions

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.