Use the new dash-plugin:// URL scheme

- Platform detection just got a whole lot better as it now uses the new
dash-plugin:// URL scheme introduced in Dash 1.9.3. This URL scheme is
documented at http://kapeli.com/dash_plugins.
- Platform detection is now able to recognise C, C++, Objective-C and
Objective-C++ files and enable the appropriate docsets for them.