Pass arguments to adapters

Although Saddle permits the specification of arbitrary Faraday adapters, there's presently no way to pass arguments to the initializer. This pull request extends the `:http_adapter` client option to specify both in a backward-compatible manner. Now it's possible to make good use of all third-party adapters. ;)

```ruby
def self.http_adapter
  {
    :key => :thrawn,
    :args => [{
      :strategy         => :synapse,
      :down_failures    => 2,
      :down_retry_delay => 30,
      :max_requests     => 100
    }]
  }
end
```

*The decision to use `:key` to designate the registered symbol of an adapter middleware was motivated by the internal name given to it in Faraday. See* https://github.com/lostisland/faraday/blob/master/lib/faraday/rack_builder.rb#L99.

/cc @mLewisLogic