Add check_on_save toggle to turn on and off check_on_save on the fly

I took a crack at allowing the user to toggle check_on_save from the menu.

Use case would be a user is looking at someone else's code but not with the intention of editing/linting/fixing. If the other person's code does not meet Rubocop's formatting then likely everything is highlighted.

![screen shot 2014-02-01 at 10 15 48 am](https://f.cloud.github.com/assets/4021179/2057886/cb6c926c-8b53-11e3-8964-ca7c194eecd4.png)

The user can quickly toggle the check_on_save option whenever their project or their desire to auto_lint change.

![screen shot 2014-02-01 at 10 17 45 am](https://f.cloud.github.com/assets/4021179/2057892/1d739740-8b54-11e3-8cba-7ec2531d703d.png)

Let me know if you think of a better way to do this!