Adds subcommand suggestions for apparent typos.

[clconway]

This adds a dependency on github.com/agext/levenshtein to compute the edit
distance from the given subcommand to the program's registered subcommand names.

Fixes #24

[clconway]

@adjackura Ping?

[adjackura]

Sorry for not setting this earlier. I'd prefer not to add dependencies to this package as it is critical for a number of systems and just adding in typo help doesn't seem like a big enough reason to change that right now.

[fgm]

One thing which could be done to support the use case without adding dependencies is support a generic catchall handler, as suggested in #30 : that way anyone wishing to add a levenshtein (or whatever) approximate matcher could add it on its own without having to fork this package.