Opening a file from commandline should optionally open in existing LightTable instance

[cldwalker]

With <= 0.7.2 and node-webkit, opening a file from a commandline e.g. light FILE opened the file in an existing LightTable app if one existed. With 0.8.0, each commandline invocation opens a new LightTable app every time. It would be nice to have this functionality toggleable so a user can choose if they want to open a file in a new app or an existing one

I checked out before the electron cleanup (#1957) with 686c9b1 (and by modifying electron version to 0.23.0) to see if this worked the old way but it didn't. I also tried when atom-shell landed (8b9b4c9) but was unable to immediately build from source.