Previously, directive options blocks have been parsed using a "full" YAML parser.
This is unnecessary, and problematic since only a mapping of string key to string value is required,
not any of the other YAML constructs; sequences, flow style, anchors, aliases, tags, ...,
It required addition handling of non-string values (failing or converting back to strings),
and could also cause confusion when values were not parsed as strings (see e.g. #712).

This commit, introduces a new "restricted" YAML parser (adapted from pyyaml),
which will maintain back-compatibilty with existing options blocks, but will only parse mappings of string keys to string values (including multi-line strings and comment parsing).
Parsing errors are also improved, by reporting the exact line on which the error occured.

closes #382