add Base16 color scheme

This was generated using https://github.com/mbadolato/iTerm2-Color-Schemes/blob/master/tools/xrdb2Xresources.py
from a xrdb file, which was generated by using https://github.com/mbadolato/iTerm2-Color-Schemes/blob/master/tools/iterm2xrdb
on https://github.com/chriskempson/base16-iterm2/blob/master/base16-default.dark.256.itermcolors