Support for extra dictionary paths

[rrthomas]

As discussed by email, please add a preference to supply a list of dictionaries to search.

@kevina, you suggested:

I think it should be local-dict-dirs and be a list of directories to
search first. It will be an list option (see manual). I think Aspell
firsts looks on the directory the dictionary is found for data files,
but this needs to be verified; if not, than a local-data-dirs might
also be needed.