ANSI escape format identifier.

Add format identifier for ANSI "CSI n [;k] m" escape sequences that have the following format:

fmt.Sprintf("%c[%dm", 0x1B, i)

These are used for setting terminal graphical options like text color, text background color, etc. 
Check [wiki](http://en.wikipedia.org/wiki/ANSI_escape_code) colors section on that.