Support %YAML 1.2 directives (#172)

This will simply allow `%YAML 1.2` directives additionally to `%YAML 1.1`.
There is no change in behaviour.

See also #20

No changes are needed regarding tag directives.

In YAML 1.2 tag directives are for the following document only.
This is already implemented like that in libyaml.

We would rather have to fix the code if we want to have the correct behaviour
(global directives) in YAML 1.1. This would be a bit more complicated, as we
would have to save the default version and the current version in the parser
object.

New passing parser tests:

* 27NA: Spec Example 5.9. Directive Indicator
* 6ZKB: Spec Example 9.6. Stream
* 9DXL: Spec Example 9.6. Stream [1.3]
* RTP8: Spec Example 9.2. Document Markers

New failing error parser tests (before they were errors for the wrong reason):

* EB22: Missing document-end marker before directive
* RHX7: YAML directive without document end marker