Plugin methods API issues

Obsevations:

1. The analytics instance is provided as a field instance on this when calling a method - this is shown in examples but not documented
2. The analytics instance is provided as an additional parameter beyond the supplied ones when calling a method - this is not documented anywhere from what I can tell, and it could cause some bugs. For example I was making a method called gtag to push arguments into the Google Tag Manager but it has to just generically push all its arguments - the extra argument could have caused some confusion here.
3. Plugin config is typically passed to all the event handlers on a plugin, but isn't passed into the method implementations, so I can take advantage of those options in the method. For example, if I wanted to use the dataLayerName option in a method, I can't do that.

Potential improvements:

1. Update the docs to clarify what is passed into these things
2. Make the config available somehow as well (maybe also on this although this-based APIs are kind of weird IMO)
3. Maybe the methods could be a function that takes an object with instance, and config, and returns the method implementation functions bound to that config and instance from scope? That could help get around the issue with passing in config and avoid using this to supply arguments

Start by locating the plugin methods API and the examples referenced in the issue, then trace how instance, supplied arguments, and plugin config reach event handlers and method implementations. Before coding, clarify whether the goal is documentation only or an API redesign; done should include an agreed behavior for config access and documented argument handling.