Using Groovy as script language instead of Expression

I am going to use scripted field to get week of year of a date at Kibana 4. I use Groovy to write the script. However, I find that the default lang of scripted field is expression. That is ,"lang":"expression".
Is there any way to change it to groovy?

Groovy scripting is not currently supported in Kibana. Sorry

Is there any plan for Kibana to support it?