Support single- and double-underline

[hubgit]

e.g.

<underline>foo</underline>
<underline underline-style="single">foo</underline>
<underline underline-style="double">foo</underline>

text-decoration-style: double isn't supported in enough browsers, so need to use border-bottom:

<u>foo</u>
<span style="border-bottom:1px solid">foo</span>
<span style="border-bottom:3px double">foo</span>