Add helm-dash-async-install-docset function

[davidshepherd7]

Url fetching and tar unpacking are both done asynchronously.

Fixes #92.

It works on my machine!

[davidshepherd7]

I created a separate function rather than modifying helm-dash-install-docset because I'm not too confident in my emacs-async abilities, and because I don't know how to write automated tests for these changes.

If it seems to work for everyone then we can rename helm-dash-async-install-docset etc to replace their synchronous counterparts.

[davidshepherd7]

@kidd Did you forget to merge this? You said you were going to merge in the issue thread :+1:

[kidd]

oh sorry. Totally forgot. Real life is interfering lately on my dev-life

[davidshepherd7]

No worries :)