New formula: rainbarf

CPU/RAM/battery stats chart bar for tmux (and GNU screen):
![screenshot](https://a248.e.akamai.net/camo.github.com/3717259cc0d9d0034f07a819ad20fc5d180e1b75/687474703a2f2f692e696d6775722e636f6d2f6d78747a3732562e706e673f31)