Parse HTML <code> as monospace format block

The in-line element <code> is currently ignored when parsing HTML.
It could be converted to a FormatBlock with format MONOSPACE, similar to how it is done for <strong> with BOLD or <em> with ITALIC.

Rendering a MONOSPACE format block in HTML currently results in a
<span class="monospace">...</span>, which does nothing if the class has no CSS style.
Instead it should be rendered as <code>...</code>.

The following should then be identical:

HTML

<code>foobar</code>

XWiki 2.1

##foobar##