Win installer/uninstaller fixes

 - renames `atom.exe` to `mapbox-studio.exe` so that in upcoming releases we can predictably check if `mapbox-studio.exe` is already running
 - starts checking if `mapbox-studio.exe` is already running and blocks installer/uninstaller until it is not detected as running
 - adds alternative 7z archive of build for windows (will test this more later). The idea is that this would be for power users wishing to run off a usb stick
 - moves a variety of things out of the nsis installer into variables that need to be passed to `makensis`
 - adds a script that can be clicked in order to build a fake installer to test locally on windows (NOTE: I say fake because the code it installs will not run - it is only useful for testing changes to the installer script without needing to push to travis)