Support codesigning dmg images for macOS ^10.11.5

[sethlu]

Address that the identity of dmg file when downloaded from a web server cannot be confirmed. codesign of macOS ^10.11.5 may sign dmg images and electron-osx-sign may provide a similar procedure for signing those files to that for signing app bundles.

Ref: #64 (comment)
Technical note: https://developer.apple.com/library/prerelease/content/technotes/tn2206/_index.html

[develar]

electron-builder 19.26.0 is able to sign DMG. Nothing to contribute to this module — is it simple one-line command (part of DMG creation).

[sethlu]

@develar cool! 😸 The issue dated back is about that generated dmg files need signing the same way as an app bundle, or gatekeeper will complain about unrecognized developer. So this feature request was trying to minimize procedure of setting up the same signing identities and passing them to codesign... Great to hear it's already made available in electron-builder!

[alirezavalizade]

@develar electron builder is not able to handle the notarize or hardedRuntime!