Support UTF-16 and UTF-32 input

[flyx]

YAML 1.2 requires UTF-16 and UTF-32 support. Currently, only UTF-8 is processed.

[flyx]

Note: As long as std/encodings says that

Warning: UTF-16BE and UTF-32 conversions are not supported on Windows.

this can't be fully implemented unless NimYAML implements its own encoding conversion, which I would rather avoid.