Add composite dictionary support (gsoc2013cdict branch)

[rrthomas]

This would be nice to have in a release.

[rrthomas]

I've some some work on this at https://github.com/rrthomas/enchant/tree/compdict

Unfortunately, the code in the gsoc2013cdict branch appears to be incomplete: the logic for setting up a composite dictionary provider is there, but the methods to deal with checking multiple dictionaries for words remains stubbed out. I have emailed the author to see if they can help.

[rrthomas]

See #349 for motivation for this work.

It would be nice if composite dictionaries were handled transparently, with no need to change the API, simply by recognizing composite language tags in the parser. This would then allow the CLI front-end to support multiple dictionaries with no changes. One question is whether the more natural comma separator could be used instead of the current colon; I see no reason why not.