request to add new message type, StringMultiArray.msg

Hi,

we uses array of string data type internally and would be nice if this type is included in official std_msg type.

we're using this type of message, for example obtaion speech recongnition result candidates.

https://svn.code.sf.net/p/jsk-ros-pkg/code/trunk/jsk_smart_apps/jsk_gui_msgs/msg/VoiceMessage.msg