Allow to specify custom build-version for electron-packager

[antelle]

Currently build-version for electron-packager (i.e. build number) is returned from env variable here. I'm using build number as commit sha from gitinfo grunt task. That would be great to allow specifying custom build number as builder.build option.
Workaround:

require('electron-builder/out/platformPackager')
    .PlatformPackager.prototype.computeBuildNumber = function() { return ...; }

[develar]

In progress. Do not send PR — there is some TS design issue (nullability). Fix will be today/tomorrow.