Disable tracing without deleting the zipkin endpoint

[devinsba]

This feature was nice in the firefox extension because it allowed us to only trace when we really wanted to but you also don't have to keep inputting the endpoint over and over

[eirslett]

Actually tracing is always enabled, as long as the plugin is turned on :-P
It would be nice to enable it when the zipkin panel is open, and disable it when it's closed.
Do you want to try a PR? :-D

[devinsba]

I can take a shot, not a javascript dev though so it might take a bit of time to get any it done well