Ignored files are excluded unless --force is passed on the CLI (fixes #813)

This PR implements the suggestion in #813 -- to exclude ignored files passed on the CLI.

To override this behaviour and lint all files passed in regardless of their ignore-state, I've added a flag `--force`.

This is still WIP, since there's no documentation of the feature yet, but I didn't want to spend time on that when the interface was not yet agreed upon.

One other thing which I think is missing is some form of alert to users when one or more ignored files were passed in. Without that, it would be quite possible that someone would run `eslint my-ignored-file.js` expecting it to be linted, and receive no errors.

As a point of reference, when ignored files are passed to `git add`, it will always exit immediately. I don't think this is useful for eslint though, since the purpose of this feature is to allow a list of files to be passed in and a subset to be linted.