feat: allow plugin deregistration from videojs

[brandonocasey]

Currently you have to get the base plugin class and then deregister from there, which is a bit weird since you can directly register a plugin from videojs. This pr allows plugin de-registration from videojs as well.

[gkatsev]

My main concern is about the weird state we can get into with the way deregistration is currently set up: if you have a registered plugin, invoke it, and then deregister. Should we dispose active instances of the plugin? or just let it be?

[gkatsev]

Also, I didn't realize that the deregisterPlugin was available on the Plugin object, I would've preferred it not be visible at all

[brandonocasey]

I think deregisterPlugin should only stop future players from getting said plugin. Plugins that are already on a player should probably be dealt with manually.

[gkatsev]

Maybe that's fine? Let's leave this to marinate for a bit.

[thijstriemstra]

An addition to the plugin documentation illustrating usage of this feature would be helpful.

[thijstriemstra]

Im not native english speaker but unregister instead of deregister sounds better to me.

[gkatsev]

They're synonyms but as this person outlines https://english.stackexchange.com/questions/25931/unregister-vs-deregister
deregister has the slight connotation that something was previously registered where-as unregistered can imply that it was never registered in the first place.

Also, given that Plugin.deregisterPlugin already exists, I would say we should just stick with it.