Suspended portal targets

[HoldYourWaffle]

Note that this PR is based off #340, as I couldn't set up a dev-environment with the outdated versions.
Let me know if I need to rebase this on develop.

I ran into trouble when using portals to implement custom "actions" for each route in a "toolbar".
When a user is navigating between routes, the page component (with <portal>) is immediately unmounted, which means that the portal-ed "actions" are immediately switched. This makes sense, but due to various animations and transitions this led to an extremely jarring experience in my application.

I can't (easily) trick vue-router into not immediately unmounting my portals, so instead I need portal-vue to wait until all the bells and whistles have completed.
Initially I tried to manually use the Wormhole, but that didn't work out unfortunately. I don't remember the exact reason, I think it had something to do with not being able to create VNode's from $ref-elements, but it was a giant mess regardless.

To fill this void I introduced a new mechanic to the <portal-target> component: suspended.
While suspended is true, all teleportation will cease. The portal-target render function will keep re-using cached children, only recalculating once suspended is no longer true.

Based on my own use-case, which the implemented demo mimics, this implementation seems to work perfectly :)
I'm not that familiar with portal-vue's codebase though, so I wouldn't be surprised if this opens up wormholes for bugs in other places.

I'm eager to hear what you think of this feature!
Also, if applicable, please let me know what additional documentation/demo's should be adjusted.

[HoldYourWaffle]

In case anyone is interested, I published this as @holdyourwaffle/portal-vue while awaiting this PR.