Enable support for Zeal on macOS

[andreas-zeller]

On a Mac with Zeal installed (but not Dash), Ctrl+H has no effect. This is because of "dash.js" only supporting Dash on macOS. To invoke "Zeal" on macOS, one needs the same options as on Linux, e.g.

const OSOptions = {
    darwin: 'zeal',  # Was "open -g"
    linux: 'zeal',
   ...
};

Suggestion: Detect dynamically whether Dash or Zeal is installed, invoke one of them. Or have a setting to choose between them.

[Realetive]

Any updates?

@andreas-zeller, is it possible to change local OSOptions manually?

[nezorflame]

@deerawan any updates?