support open online doc (docs.rs)

[xxchan]

First of all thanks for the awesome tool!

The issue is:

Dash couldn't redirect you to the online version of the page you were viewing, because it doesn't know where to find that page.

https://kapeli.com/docsets#onlineRedirection

Why I want this:
On docs.rs, the top toolbar is sometimes helpful (other versions, dependencies, feature flags...)

[Robzz]

Thanks for the link, I didn't know about that feature. That should be do-able, adding to the TODO list.