Add loading spinner (loadingGif) to electron-winstaller-fixed options

[atietje]

createWindowsInstaller allows to specify a file that will be used instead of the default loading spinner. This option is not used in WinPackager.packageInDistributableFormat.

I suggest to add loadingGif: this.metadata.loadingGif to the options in function packageInDistributableFormat and add the path to the development package.json like
{ "build": {"loadingGif": "./build/install-spinner.gif"} }

[develar]

suggest to add loadingGif: this.metadata.loadingGif to the options

This option should be listed not in the metadata root ., but in the build.win. It is supported already. See https://github.com/electron-userland/electron-builder/blob/master/docs/options.md

add the path to the development package.json like

Yep! Definitely should be added.