Add a supported way to get the current list of plugins

[nedbat]

Is your feature request related to a problem? Please describe.
The django-coverage plugin wants to examine the current list of plugins in its tests. It also wants to know whether a plugin is enabled or disabled.

Describe the solution you'd like
Please provide a method on Coverage to get the current plugins. Provide a method or attribute on the plugins that indicate whether they are enabled or disabled.

[devdanzin]

This would only need a way to retrieve Coverage._plugins.names, as CoveragePlugin already has a _coverage_enabled attribute, right?