[editing] JavaScript implementation

Hi,

I'm really benefiting from all the hard work here (
https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html) and I think it's
a great subject to address.  Specifically, I've really found the JavaScript
implementation (
https://dvcs.w3.org/hg/editing/raw-file/tip/implementation.js) a much
needed tool for in-browser editing.

While working with that implementation, I noticed what I would call a bug.
Specifically, line 2278 returns "undefined" for the "strikethrough" and
"underline" commands, causing unnecessary "span" elements to wrap
selections when executing some commands.  I imagine either the
"relevantCssProperty" for these two commands should be set to
'textDecoration' or perhaps there are implementation-specific issues and it
should be set to "null".

I hope this is the correct channel to provide this feedback; I did not see
a better-fitting place to comment on the page.

-Michael