GH-240 electron: Allow passing all platform options to electron-builder

[RaddishIoW]

Platforms affected

Electron (Mac/Win/Linux)

Motivation and Context

Fixes #240.
electron-builder allows options to be passed which enable auto-updating, among other things.
Cordova-Electron was unnecessarily restricting the passing of options in the platform configs which prevented the use of these extra configuration options and features of electron-builder.

The tests for cordova-electron were not catching this as they only tested each individual unit rather than the integrated code-flow.

Description

Removing the function __validateUserPlatformBuildSettings and it's call in lib/build.js enables the use of these extra config keys. Any errors caused by incorrect config keys should be caught by electron-builder itself, negating the need for this check.

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary