Add Retweet/Favorite Counts Format String

Format string of retweet/favorite count number is unavailable in current format string items, but I think it should be very usable if add them. ˊ・ω・ˋ
![twmode-favcount](https://cloud.githubusercontent.com/assets/1370070/5009494/7310b25e-6aa2-11e4-8ce1-0b3fd4919a2a.png)