Provide a python interface equivalent to fst2strings

[santhoshtr]

hfst-fst2strings accept multiple options related to the prefix string in input and output and implemented using the callbacks in extract_path. This feature is not exposed in python binding. It would be useful for various usecases that require string extractions from a system.

My usecase is this: I have a morphology analyser for Malayalam(https://gitlab.com/smc/mlmorph/). Malayalam is a heavily agglutinated, inflected Dravidian language. Given a verb or noun root prefix, I would like to extract all the generated words that match the given prefix - may be in input or output. This will also help in the usecases where a "look-ahead" or "auto-complete" like features are required.

PS: In case there are alternate ways I can achieve the above usecase, please let me know. Thanks in advance!