add helm-dash-deactivate-docset function

[areina]

Hey @Dickby, thanks for your contribution. I never needed something like this, do you think it is really useful? Which is your use case?

Thanks!

[Dickby]

I know that it's possible to activate docsets mode-local, but I just activate a docset when ever I need it, this way I can search the active docset in other modes like dired too. I code in different languages, and if I switch to another project, sometimes it's handy to deactivate active docsets. Since it's possible to install a docset without activating it, I think deactivating a docset makes sense too. Thanks for the package by the way.

[kidd]

Huge amount of workflows :)

Thanks for your contrib!