Fatal Logging modes

[Casao]

Adds support for fatal logging level - fixes #141

[dominikh]

This makes me wonder why we need both fatal and error. error, if it worked correctly, would look exactly like fatal. fatal, to me, has the connotation of not permitting any further code to execute, so maybe it should raise an exception instead? Only that then we wouldn't need fatal at all, the user could just use raise (and we'd even display that exception nicely formatted.)

[Casao]

From my experience, fatal logging is just logging, and the exit is left up to the calling code. Fatal is basically some sugar to state why it shut down

[dominikh]

Well, where I come from (Go), log.Fatal will actual exit the program, too.

Anyway, if "fatal logging is just logging", we should still determine how it would be different from "error logging", which would also use !! and be red. If they look exactly the same, we don't need both. Now the question is whether we should make one of the two look different, or if one of them should be dropped completely.