Support v-once directive

[danywalls]

Should be able add option to use v-once and only listen for visibility 1 time ?

[johndatserakis]

Yea honestly that would be awesome - there's a really basic use-case of wanting to show an animation a single time - that would be totally useful.

[johndatserakis]

Dude you are a beast! How do you even find time for stuff like this with all the work you do? Much appreciated!