Pass safeMode and devMode load settings to new windows

Previously, when you opened a new window from safeMode or devMode, the mode settings were not persisted. This has always annoyed me. Now, when you open windows from safe mode, they are also safe mode windows, and if you open dev mode windows from a dev mode window, they are dev mode windows.

Closes https://github.com/atom/atom/issues/4690
Should this be changed as well for consistency?