Support rules to specify unhandled cases

[ribose-jeffreylau]

E.g. combinations not supported by the transliteration system

Originally: https://github.com/interscript/lcs/issues/2#issuecomment-578096993

[ronaldtse]

A "default" conversion for things that fall out of the target character class.

For example, in a Japanese (script) to Latin (script) conversion, if we miss a particular Japanese rule, the leaked Japanese character will violate the target character class. We will need a way to indicate what to do in such case (because we don't want to break the reader).