Settle the directive syntax

[janbrohl]

by @dhellmann

[dhellmann]

I think I like the approach of using different directives based on the format of the input best. That allows us to have format-specific options, which we know we need in a few cases. It also makes it easy to document the directives and their options cleanly.

If you agree, we can close this question and open a ticket to remove the old legacy format before we release 1.0.

[janbrohl]

Actually, we could use tha plain directive for autodetection like #40