Fix plugin api

There didn't seem to actually be a `.use(fn)` method exposed, so I added it... but it's a bit opinionated due to my use case and simply adds all plugins to run *before* the internal plugins. This is because my use case need to add new properties to `metalsmith.metadata()` prior to the internal plugins.

cc: @ianstormtaylor
This lets us ignore any data automatically added by plugins via `Khaos.prototype.use` and not just limit it to the internal `date` and `basename` plugins.