enable use of gzipped dictionaries

[flachs]

this is a rewrite of the dictionary file search code to support dictionaries stored in different formats as indicated by the filename extension. ".hz" for hzip, ".gz" for gzip and no extension for plain.

this requires requires a version of hunspell that can actually load gzipped dictionaries. flachs/hunspell is one such version.

[ZJaume]

Is this planned to be merged at any point? I'm interested.