Add support for YAML comments as an option

[cowtowncoder]

Regrettably it looks like YAML does not support C or C++ comments but only scripting comments (hash sign until end of the line). While this should not cause problems with JSON per se, there are some cases where this is problematic: specifically, when YAML is viewed as a superset of JSON; in which case one has no common comments (granted, comments are not in standard JSON, so it is superset of standard JSON).

Anyway: ability to enable handling of "hash comments" would be useful and simple to implement; quite similar to how C++ comments are handled. As such, it would make sense to add in 2.2.

[cowtowncoder]

Hmmh. Turns out that highly optimized implementation of decoding makes this more difficult than I thought -- tight loop for skipping white-space is only called for characters below value of hash character.
Will need to see if this could still be implemented or not.

[cowtowncoder]

Implemented in the end by modifying white-space handling loop, changing to use lookup table. Hopefully no measurable performance impact.