hunskell: add basque dictionary Xuxen 5

mkDict doesn't easily adapt to dictionaries that have multiple source
files but no readme, so I did not use it. Having a "generic" function
that then has per-language quirks is a bad abstraction.

A small utility function that copies a given file to hunspell's and
myspell's dirs and a separate one that copies a given readme file would
be more useful.