Work on OS X, alongside the original Dash (also: various fixes)

The docset paths on OS X are different, and the docsets installed by Dash itself are nested in subfolders. So this PR contains fixes which ensure that `helm-dash` works for users on OS X who have Dash installed.

The only caution is that OS X users with the original Dash installed *might* find that installing docsets from `helm-dash` confuses (or even breaks) Dash. I haven't tried that out.

Disclaimer: I haven't tested this on a non-OS X system, but I haven't made any changes which I would expect to break things elsewhere. :-)