Language-specific optional characters

It would be cool to be able to define, on a per-transducer basis language specific basis certain characters which can appear anywhere in the stream but that don't effect the analysis.

This could possibly be used for soft hyphen,[1] for tatweel[2] and various kinds of zero-width joiners/non-joiners and floating punctuation symbols, e.g. Armenian interrogative signs.[3]

1. Man­You
2. رحــــــيم = رحيم
3. Ինչպե՞ս

There are many open questions regarding what exact form this should have and what kind of behaviours we should support.