Added excludeKeys option to prevent functionality

[mrwhy-orig]

Should work for #53.

usage: Vue.use(VueShortkey, {prevent: [{name: 'input', excludeKeys:['f1','f2','f3','f4','f5','f6','f7','f8','f9','f10','f11','f12','up','down']}, {name:'textarea', excludeKeys:['f1','f2','f3','f4','f5','f6','f7','f8','f9','f10','f11','f12','up','down']}]});

Maybe we should add some constants and or a range method.

[fgr-araujo]

Hi! I'm testing..

[fuzzybaird]

So what happened with this feature? I still think it would be pretty cool.

[bbeng89]

Any update on this one? Looks like exactly what I need.