Exclude attributes starting "on" on HTML elements

Because:

* It doesn't work well with TW5's refresh mechanism, which relies on
being able to regenerate any portion of the DOM as required; this
frequently causes inline handlers to be re-executed at unexpected times
(see
http://tiddlywiki.com/static/TiddlyWiki%2520for%2520Developers.html)
* It mixes TW5 version-specific JavaScript with user content
* In multiuser environments there is a security risk to importing or
viewing tiddlers you didn't author if they can have JavaScript in them