keyboard shortcuts won't fire with modifier keys

I had particular problems with Ctrl-N (open new window/tab)
interfering with N (go to next unread item). This change
disregards keyboard shortcuts if Alt, Shift, or Ctrl are used.
It also tries to disregard Win/Super/Meta, though this didn't
work for me in Firefox. Still, just the Ctrl is an improvement
for me.
Merge branch 'keyboard-shortcut-modifier-keys' of https://github.com/jarek/miniflux