Support user-added filters

Currently all filters are hard-coded into dust.js. Helpers can be user added and this is a supported extension point. Filters are useful for a different set of problems than helpers and could easily be made user-extendable in much the same manner.

I would suggest that filter names of the form x- be designated for user-extensions leaving the rest of the namespace for language expansion. Otherwise we would always be facing a backward-compatibility risk.

To increase the flexibility I would suggest passing context on down to the filters as mentioned in Issue 204. This gives them more flexibility. A benefit of filter extensions is that the resulting object can still be passed as a param, etc unlike when a helper is used.

Start by locating the filter implementation in dust.js and compare it with the existing user-added helper extension point. Review Issue 204 for the proposed context handling. Done means user-defined x- filters can receive context while preserving the existing filter namespace and allowing their results to be used as parameters.

Written by the indexing model from the issue text.