Conform supported encodings to Encoding spec

This change conforms the set of supported encodings for the parser to
the requirements in the Encoding spec. Specifically, this restricts the
set of encoding names and labels to those listed in the table at
https://encoding.spec.whatwg.org/#names-and-labels, and the statement:

> The table below lists all encodings and their labels user agents must
> support. User agents must not support any other encodings or labels.

Author: sideshowbarker