Allow people to use ' entity (sinqle quote character: ') in wikitext.

Allow people to use ' entity (sinqle quote character: ') in wikitext.

It probably wasn't included originally, as its not part of html4,
but was introduced in XML/XHTML1. Since we do all sorts of weird stuff
with ' characters (italic/bold) its probably good to allow lowly users to
have an easy to remember way of escaping it.

There's also a whole bunch of entities in html5 that we should maybe consider
decoding as well.