Reduce ViolationSeverity to two members: Warning and Error

[jpsim]

As discussed in #106, 5 severity levels is overkill, especially since Xcode can only (really) display 2: warning and error. So this should simplify things quite a bit.

[segiddins]

We could use a third level as note, which should show up in the gutter but not cause build failures.

[jpsim]

I really don't know when that would be used in practice, I don't think any of SwiftLint's current rules would be appropriate for representation as a "note".

[jpsim]

It's trivial to add .Note as a case of ViolationSeverity, so we can always add it later if we find a use for it.

[cuddergambino]

There's a use case I discovered:
Have approaching_expiry_severity: note and expired_severity: warning to only warn of expired todos and note approaching expiries.

I originally was going to make a new request for a none case, but found this issue similar enough. Should I make a new request for this?

[sanguish]

This would be a great solution for these. Since I don't want to see them as a warning before hand.

It's missing from the documentation how to set approaching_expiry_threshold