look ahead to include closing tags

I tried this on a change from
`chrome.runtime.sendMessage({ what:"label", stuff }, function() {`
to
`chrome.runtime.sendMessage({ what:"label"/*, stuff*/ }, function() {`

Using webdiff this displays as:
`chrome.runtime.sendMessage({ what:"label"pan class="hljs-comment">/*, stuff*/ }, function() {`

When stepping through the html and text in advanceOne() I think it should look ahead to include the closing hljs `</span>` tag. Otherwise the next time around will get out of sync and result in funky markup.