Add support for multiple key strokes combinations

Add support for multiple key strokes combinations, for example,

    key('ctrl+r+s', function(){ alert('stopped reload!'); });
    key('ctrl+shift+r', function(){ alert('reload!'); });
    key('command+alt+p', function(){ alert('pause!!'); });
    key('command+shift+w+p', function(){ alert('do something!!'); });