<~ only allows 1 callback

[Danny02]

Hi, I just stumbled over a little problem I have with your little callback extension for event attributes.

with the method in
https://github.com/lihaoyi/workbench-example-app/blob/todomvc/src/main/scala/example/Framework.scala#L107

one can only ever register one callback, because old ones get overridden(i.e. can't register a function to "onclick" and "ondblclick")

[lihaoyi]

Yeah, it's more a rough proof of concept than anything that's worth seriously relying on. If you want to improve it you can send a PR, but I'm probably not going to put in much more effort into API design and polishing since it's just an example to get started =P

[lihaoyi]

Should be fixed by your PR