Make colours optional via --no-colours / --no-colors CLI switch

Related to #405 (but doesn't address the color-reset issue on Windows)
This will be undefined at the time you load the `log.js` so I have a feeling it won't work, in addition, you're lasing through a primitive rather than a function (in contrast to `isRequired`) so changing its value later after the config is read, doesn't affect the logger (if I'm correct - sorry, reviewing code from a mobile device!).