Create a UnicodeUnescaper.
The constructor takes a list of options, only one type of which is currently
available (whether to allow, error or ignore the semi-colon on the end of a
numeric entity to being missing).
For example, to support numeric entities without a ';':
and to throw an IllegalArgumentException when they're missing:
Note that the default behaviour is to ignore them.
Translate a set of codepoints, represented by an int index into a CharSequence,
into another set of codepoints. The number of codepoints consumed must be returned,
and the only IOExceptions thrown must be from interacting with the Writer so that
the top level API may reliably ignore StringWriter IOExceptions.