adds param for client certs

adds an option to the command parser for client certicates

plugins/commands/box/command/add.rb

@@ -19,6 +19,11 @@ module VagrantPlugins
               options[:insecure] = i
             end
 
+            o.on("--cert certfile", String,
+                 "The client SSL cert") do |c|
+              options[:client_cert] = c
+            end
+
             o.on("--provider provider", String,
                  "The provider that backs the box.") do |p|
               options[:provider] = p
@@ -40,6 +45,7 @@ module VagrantPlugins
             :box_url      => argv[1],
             :box_force    => options[:force],
             :box_download_insecure => options[:insecure],
+            :box_client_cert => options[:client_cert]
           })
 
           # Success, exit status 0