adds proxy configuration option

[owainlewis]

A pull request that adds client configuration to the client allowing users to set proxy options if required.

(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)

[owainlewis]

Apologies. I didn't realise someone else already had a PR in for this. Feel free to close. Thanks. : )