Per-handler configuration for plugins

[kele]

Plugins should be configurable per handler. This is especially important for plugins which use interceptors.

Examples:

1. mux.Handler(..., csp.Disable()) -- CSP plugin is installed on the mux, but disabled for a given handler.
2. mux.Handler(..., cors.Enable()) -- CORS plugin is installed on the mux, but by default it is not applied to any handlers, therefore we need to enable it.
3. mux.Handler(..., auth.AllowAnonymous()) -- to change how access control is done.

This need stems from the design decision of not having per-handler plugins. The problems of having per-handler plugins (as opposed to mux-level plugins and per-plugin configurations):

• what should be the order of interceptors execution?
• overriding of plugins installed on handler level by the ones on mux level - how to remove a plugin?

Overall, having configurations seems like an easier option.