The plug specification.
There are two kind of plugs: function plugs and module plugs. A
function plug is any function that receives a connection and a
set of options and returns a connection. Its type signature must be:
(Plug.Conn.t, Plug.opts) :: Plug.Conn.t
A module plug is an extension of the function plug. It must export a
`call/2` function, with the signature defined above, but it must also
provide an `init/1` function, for initialization of the options.
The result returned by `init/1` is the one given as second argument to
`call/2`. Note `init/1` may be called during compilation and as such
it must not return pids, ports or values that are not specific to the
The API expected by a module plug is defined as a behaviour by the
`Plug` module (this module).
## The Plug stack
The `Plug.Builder` module provides conveniences for stacking many
plugs into a single module.