[Feature Request] Support macOS Dash.app third-part docset locations.

[stardiviner]

On macOS Dash.app, the default main docsets are downloaded in "/Users/stardiviner/Library/Application Support/Dash/DocSets". But Dash.app also downloaded third-part docsets in other locations:

Cheat Sheets     Data             GitHub           Stack Overflow   library.dash
Clojure DocSets  DocSets          License          User Contributed

Can dash-docs.el make variable dash-docs-docsets-path to be a list of paths for search docsets?