Add mailto support in openExternal function

[fbricon]

I tried something like window.ddClient.host.openExternal('mailto:[email protected]'), but nothing happens when invoking that bit.
I found electron/electron#1083, that might be related (or not)
I tried with and without having a Mail client installed on my mac.
There are no errors in the console.

[gtardif]

Thanks for the feedback, the SDK currently supports only opening http:// or https:// urls.
We'll at least add clear error messages in the console for URLs that are not supported, and see if we want to expand the list of supported urls to include mailto:// urls

[benja-M-1]

Since version 4.9.0, using another protocol than HTTPS or HTTP an error is thrown by the openExternal function.

As of now, this becomes a feature request :)