added launch at startup functionality

[matthewauld]

Implements #1665
Alright! So, this is not fully tested but:

• works on mac
• don't have a windows box to test, but I followed all the instructions to ensure it works with the auto-updater.
• doesn't work when running from source (but I don't think you want it to?)
• linux works, but I cannot seem to find the correct executable path. both process.execPath and app.getPath('exe') give you the file stored in /tmp/.mount_Beakerxxx which seems to have something to do with the builder? I did a bunch of searching, but cannot find the solution. I am sure it is straightforward and if it is easy, you just need to swap out process.execPath on line 889 of browser.js

If you need changes, just let me know!

[pfrazee]

Thanks @matthewauld! I'll need to test this out when I'm close to the next build and testing on platforms.