adds proxy configuration option

A pull request that adds client configuration to the client allowing users to set proxy options if required.

```clojure
(def auth-with-proxy {
   :access-key ""
   :secret-key ""
   :client-config {
     :proxy-host "www-proxy.bbc.co.uk"
     :proxy-port 80
   }
   :endpoint "https://dynamodb.eu-west-1.amazonaws.com"})

(list-tables auth-with-proxy)
```