Add formatter option to strip ANSI escape codes

I tend to add colours to my log messages to make them easier to read during development (plus I'm a sucker for pretty console output), but it's not ideal to keep the colours around when logging to a file or database.

I've added a new `strip` option to the formatter that runs the `message` and `args` through [chalk.stripColor](https://github.com/sindresorhus/chalk#chalkstripcolorstring). It also disables `colorize` if both are enabled at once - cause that's just silly.

Thoughts?