Introduce Tesla.Client with pre/post middleware

[teamon]

This change is backward compatible:

• Tesla.build_client API stays the same
• one can still use simple function as a client

Tesla.Client struct can hold any custom fun (as before)
but can now hold pre and post middleware lists.
pre works the same way as one given to build_client.
post middleware is injected right before adapter.
It can be used as a regular middleware but can be also
used to dynamically override adapter in runtime.
This is mostly useful in testing:

defmodule MyAPI do
  use Tesla

  def help(client \\ %Tesla.Client{}) do
    get(client, "/help")
  end
end

client = Tesla.build_adapter fn env ->
  %{env | status: 200, body: "new body"}
end

assert %{body: "new body"} = MyAPI.get(client)

[amatalai]

I believe these two should be referenced here #94 #97 :)

[teamon]

As far as #94 is concerned there is one more way to do this (and I'm surprised I haven't thought about it earlier)

defmodule Cached do
  use Tesla

  adapter :cache

  def cache(env) do
    cond do
      String.ends_with?(env.url, "/cached") ->
        %{env | body: "cached", status: 304}
      true ->
        Tesla.run_default_adapter(env)
    end
  end
end

All one needs to do is to write a custom adapter that wraps other adapter.
I've added Tesla.run_default_adapter/1,2 as part of this PR to make it more convenient to use the default adapter which can be configured globally.