Take the plugin system out of experimental

[blakewilson]

We have proven that our plugin system and model works, so let's take the experimental off of the experimentalPlugins flag in the faust.config.js and just use plugins. We should also be backwards compatible and still support experimentalPlugins but warn users that they should start using plugins now.

Acceptance Criteria

• New plugins property is added to the Faust config
• experimentalPlugins is still supported with a notice that they should start using plugins instead.
• Update documentation