Minor text formatting updates

I've just updated the site's markdown processor a bit to allow a few more HTML tags through, and added a section to the Text Formatting docs page to explain these options. The short form is that the following formatting is available only by using HTML (some of these may be added with markdown syntax eventually):

• strikethrough - <del> tag
• strike replace - <del> and <ins> tags
• ^superscript - <sup> tag
• _subscript - <sub> tag
• Tables - using <table>, <tr>, <th>, and <td>

I think at this point we should probably have almost all of the wanted formatting available, but there may still be a few missing.