Allow a pubsub to be passed into the PubSubPlugin or initialized on every new plugin

[spentacular]

Currently pubsub is private to the pubsub plugin, which is always initialized with a shared instance of pubsub:

player/plugins/pubsub/core/src/plugin.ts

Line 36 in 6e7bb7d

this.pubsub = pubsub;

Instead, allow an instance to be passed in to allow interfacing with other common pubsubs (which would require having it's own expression as well).

Also, have the constructor instance of pubsub be a new instance for every initialization of the PubSubPlugin.