Add 'knob' gutter mark theme

Before I started using Anaconda I shortly used SublimeLinter with pylint. I really liked the red/amber 'knob' gutter mark theme so I adapted it for Anaconda. I added the third white icon.

Hope the colour choice make sense - red for error is clear, I wasn't sure about warning/violation. At first I thought amber will be for warning and white for violation but in practice it seemed to make more sense the other way around.