Fix github issue #23, not parsing an escape char case properly.

It turns out that unrecognized characters in escape sequences
should be tolerated and passed through. This change implements that and
also adds support for \0 in string literals:

    $ python3
    >>> import json5
    >>> json5.loads(r'"\/"')
    '/'
    >>> json5.loads(r'"\0"')
    '\x00'
    >>> quit()
    $
3 files changed