Add -s, --stealth-mode option

Only lock the screen, but don't draw anything. ..Kids, you can watch,
but.. U Can't Touch This!

From ravinrabbid's invisible mode commit.

https://github.com/ravinrabbid/i3lock-clock
commit f607ca4542ea1603762e93f8312ae9917a4b6c55
Reduce failed attempt redraw timeout

Reduce redraw timeout after an unsuccessful authentication attempt when
unlock indicator is always visible.
Add version suffix