Add .leptonrc option for default zoom level & shorcuts for zoom

[aguytech]

Hi,

it would be wonderful to have a default zoom level & zoom shortcuts settings (like other shortcuts) in .Lepton.rc

I'm on Linux Manjaro & Ubuntu, with azerty keyboard, the ctrl+_ / ctrl+- do not works for Zoom out. Lepton v1.10.0

Thanks for this works)

[hackjutsu]

Just curious, how does zoom level work for Chrome with azerty keyboard? I believe Electron uses the same set of shortcuts in this case.