Adding underline support

Similar to how strong and italic are currently implemented.


From the WordprocessingML doc (http://rep.oio.dk/Microsoft.com/officeschemas/wordprocessingml_article.htm):

```
For example, the underline element uses the val attribute but offers more choices than "on" and "off". This example gives the text a single, continuous underline (other options include "words", "double", and "thick"):

<w:r>
   <w:rPr>
      <w:u w:val="single"/>
   </w:rPr>
   <w:t>How are you today?</w:t>
</w:r>
```