Added a propagate modifier to let other listeners catch events

[pirhoo]

This modifier allows several listeners to catch the same event.

It might solve issue #78 as it gives a neat way to bind the same key on different elements. This will work:

 <my-component v-shortkey="['ctrl', 'alt', 'o']" @shortkey.propagate="anAction()"></my-component>
 <my-component v-shortkey="['ctrl', 'alt', 'o']" @shortkey.propagate="aDifferentAction()"></my-component>

I added a few tests for edge cases.

Thanks a heap ✌️

[fgr-araujo]

I'll check!
Thank you

[DennisOng]

Ran into this today as well, thanks for this PR @pirhoo, and @iFgR for this great package!

Would you be able to tag this new release for this?

[pirhoo]

Looks like there is a typo in the new modifier ("propagte" instead of "propagate"). I'll do another pull request.