gui: Add first prototype version of GUI

[mattwynne]

Summary

Add a prototype GUI (Graphical User Interface) for Cucumber, built on electron.

Details

The new evolving, standardised, event-protocol allows us to build user-interfaces that can be used with any Cucumber implementation that supports the protocol. This PR contains a spike (i.e. there are no tests, and we will not continue to maintain the code in its present state) of a Graphical User Interface for Cucumber, build on Electron.

Motivation and Context

This proof of concept was, built for two reasons:

1. to be a consumer to flush out the events we need for transmitting results through the event-protocol
2. to test out Electron's capabilities and get people excited about the possibilities of building a cross-platform desktop front-end for Cucumber.

How Has This Been Tested?

The app is currently tested manually using the event-stream-3 branch of cucumber-ruby.

Screenshots (if appropriate):

There is a video.

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