Fix for symbol problem (e.g., with apostrophes).

Text with symbols like "&#39;" will be mistaken for issue references #1.

For example, markdown with apostrophes like "My name is 'Carlos'." will get converted to "My name is 'Carlos'.". And when gfm-linkify processes it, it will think there are two references to issue 39. This can happen to any symbol with a similar format.