add new utility function colorCategory20 to jsk_topic_tools

a function to select one color out of 20 different colors.
(use the same colors to [d3.js](http://d3js.org/))

![](https://camo.githubusercontent.com/910992baba133ee268b1b43b3ebd3b02ff708780/687474703a2f2f6661726d362e737461746963666c69636b722e636f6d2f353530372f393338383530303532315f316563343436633065395f6f2e6a7067)