Implement support for specification of pre-processing engines

[ribose-jeffreylau]

A way to specify pre-processing engines, e.g. use module ABC to preprocess (normalize, segment, tag name entities) the text before performing transliteration tasks below

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

[ronaldtse]

Requires #709 to be used here.