Cache should not be valid when lint command and arguments change

[afoglia]

I made a change to my .gitlint.yaml to refer to my projects .pylintrc, but wasn't seeing the changes when I ran git-lint. I eventually realized I needed to blow away the cache.

Ideally, the cache would realize it's expired and must be regenerated.

At the very least, until then, the Configuration section of docs should be updated to note that after any config changes, one will likely want to delete the cache.

(This is git-lint v0.1.2.)