Adding additional filters

This change enables external filters to be added to mutations without altering mutations itself. This saves time as you don' t have to maintain a separate Mutations repository for all the project specific data-types.

Simply write the filter and have it subclass AdditionalFilter.
Place the filter in your (rails)project and you can start using the newly created filter in your project right away.

Testing these filters is just as simple, just add the filter test to your project.

Note that this change doesn't alter the default workings of Mutations, its fully backward compatible.