enable cacheing?

[ssfrr]

For cells that take a long time to compute it would be great to use Weave's cache support. One way to implement it would be to add the cache=:user option when running Weave from the button, and then if the user ctrl-clicks the button it would be cache=:refresh. Using the :user option would mean that it wouldn't do any caching unless the user opts-in.

The ctrl-click behavior is sort of like what Atom uses for the git push button, where ctrl makes it a force push.