Add SubmoduleStatus post-checkout, post-commit, post-merge, and post-rewrite hooks

[jawshooah]

These hooks warn of any submodules that are uninitialized, out of date with the current index, or contain merge conflicts.

To check submodules recursively, just set flags: ['--recursive'] in the config.

Closes #145

[sds]

Since we're duplicating this helper Struct across each of these hooks, and since submodules are a git primitive of sorts, it seems appropriate to extract the regex, struct, and helpers into the GitRepo module. That would allow us to DRY up a lot of this.

What do you think?

[jawshooah]

Agreed, I've extracted the regex, struct, and helper to GitRepo. Also made recursive a config setting.

[sds]

Thanks!