Add option to show main process output in the renderer

[sindresorhus]

Can you be useful for debugging a production app, since you wouldn't otherwise easily see the logging from the main process.

@acheronfail Thoughts?

[cgalvarez]

In order to see the main process messages you can execute the built app from the command line like path/to/app/folder $ ./app_executable

[sindresorhus]

@cgalvarez Yup, but that's inconvenient. Also nice to have all the logging in one place when debugging.

[sindresorhus]

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