Extend a macro-based client with new middleware for runtime usage

[alizain]

Usage is pretty simple, helps me re-use a compile-time client with some modifications at runtime.

defmodule SomeClient do
    use Tesla

    plug ...
end

# somewhere else
[
    {Tesla.Middleware.BearerAuth, token: 1}
]
|> SomeClient.extend
|> Tesla.get!(...)

[yordis]

Hey there, do you mind adding unit tests to prove the use case and write documentation about it?

[alizain]

@yordis updated, what do you think?

[yordis]

See #612