Fixed none working keybindings with <ctrl-x> whene x is a modkey.

If a keybinding exists with a modekey 'x' no keybinding with 'x' and
additional modifiers worked.
Added command pass-through to switch to pass through mode.

The pass-through mode allows to skip all keybindings accept of <esc> and
<ctrl-c> in vimb to let the webview handle them.