Add results caching

Cache results so that linter doesn't perform cpu heavy stuff all the time. Not sure how appropriate this PR will be, posing it anyway. Will add some tests later (especially if you're willing to accept it).

Little background: we have pretty busy CI server which runs test, linting and other nice checks for us. We've found that *coffeelint* consumes **35 seconds** from build time. With cache option on average this task consumes only **800ms**, which is approx **40** times faster.

Any thoughts?