Support inline display HTML tags

[maxg]

The recursive parser doesn't differentiate HTML tags, so:

Foo <span>bar</span> baz

becomes:

<p>Foo</p>
<span><p>bar</p></span>
<p>baz</p>

This would eliminate many situations that currently require class="no-markdown", e.g. <sup> and <sub>.

Perhaps a whitelist of elements to treat specially during recursive parsing?

[maxg]

Fixed (questionably?) in 00ac2c3 for <sub> and <sup>, tracking need for more tags in #18.