Add Chef Supermarket provider

Upload Chef cookbooks to https://supermarket.getchef.com/

#### Options:

 * **user_id**: Required. The user name at Chef Supermarket.
 * **client_key**: Required. The client API key file name.
 * **cookbook_category**: Required. The cookbook category in Supermarket (see: https://docs.getchef.com/knife_cookbook_site.html#id12 )

#### Examples:

    dpl --provider=chef-supermarket --user-id=chef --client-key=.travis/client.pem --cookbook-category=Others