Adds a section on 256-colors.
Adds a section on 256-colors.

Closes #35.
References #44.

pedroha/chalk,yamii/chalk,sterpe/chalk,chalk/chalk,stevemao/chalk,cnuis2cool/chalk,chalk/chalk,sindresorhus/chalk,rsandor/chalk,joegesualdo/chalk,lukeapage/chalk,hughsk/chalk