Add Spellcheck commit-msg hook

[jawshooah]

This hook uses hunspell to check the commit message for potential misspellings, and suggests alternate spellings if available.

[sds]

I'm realizing now that calling a destructive operation on the commit message file as part of a hook run is dangerous if we were to ever parallelize hook runs.

I realize you didn't add this helper, but I also realize that this helper isn't actually used anywhere (until now).

Ideally, we would create a temporary file with the comment lines removed and run the hunspell tool against that.

However, I also acknowledge that commit-msg hooks would want to be able to modify the commit message at some point.

For now, since this hook doesn't modify the commit message, let's run hunspell against a separate temporary file. We still need to think about the GerritChangeId hook, which modifies the commit message file directly.

[jawshooah]

Good call, I've updated the hook accordingly.

As for parallelizing hook runs, any hooks that directly modify the commit message file will probably have to be run serially, which can be specified in the config as discussed in #144.