Add base16 256-color themes.

* base16-chalk.dark.256
* base16-chalk.light.256
* base16-default.dark.256
* base16-default.light.256
* base16-solarized.dark.256
* base16-solarized.light.256
* base16-tomorrow.dark.256
* base16-tomorrow.light.256
* base16-tomorrow.dark.256
* base16-tomorrow.light.256
* base16-twilight.dark
* base16-twilight.light
Update default Nichol color theme to base16-tomorrow.dark.256.