Parametric route

[Gigitsu]

This adds support for parameters in expectation url.

You can define expectation like this:

Bypass.expect_once(bypass, "GET", "/1.1/users/:user_id", fn conn ->
    user_id = Map.get(conn.params, "user_id)
    Plug.Conn.resp(conn, 200, ~s<{....}>)
end)

and the request client->get("http://localhost:8080/1.1/users/1234") will successfully match the expectation defined above.

[ream88]

I like this, @MSch what do you think?