cache not invalidated on config changes

[simon-weber]

If .gitlint.yaml has been updated since the cache was written, the output cache should be invalidated. Otherwise, you could edit the config to eg ignore an error type, but it'll still be present when rerunning.

[simon-weber]

How about storing a single file in ~/.git-lint (above the cache tree so we don't clobber the namespace) to represent the last modification of the config? If that's acceptable, I can send a PR.

[sk-]

Hi Simon, I think that's a good solution. Feel free to send me a PR.

By the way this is a duplicate of #19. So I will close it.