APIs needed to integrate macOS Dash.app with JuliaHub

[Kapeli]

Hello. I'd like to add support for JuliaHub in Dash. This is a continuation of mofeing/julia-package-docsets#2.

I need the following APIs:

• A way to search for packages
• A way to get what versions are available for a package
• A way to download the (offline HTML) docs give a package and version