Code review comment for ~bfrk/epics-base:address-modifiers

@Andrew I have addressed most of the issues you raised and made the address modifier API polymorphic.

The parsing is not yet encapsulated. There is a trade-off here: we use the same syntax as a short-cut for filters. Not sure what to do here; perhaps create a separate module just for this parser. I'll have to experiment to see if that results in a better structure.