Separate main and renderer process logic

[acheronfail]

As discussed in #10 - the codebase may be much easier to maintain if we separated the main and renderer logic into separate files. This issue will track the progress of this.

Current goals:

• Separate main and renderer process logic
• Improve tests
  • Make it easier to write tests without them effecting each other
  • Hopefully make it pleasant to write tests (we can only go so far with Spectron)

[sindresorhus]

See #19 for inspiration if you want to work on this issue.