Relaxed plugin format (proposal)

index.js

const fs = require('fs')
const path = require('path')

module.exports = function() {
  return {
    pre_pug: function(pug, dir) {
      return pug
    },
    post_pug: function(html, dir) {
      return html
    },
    mixin: fs.readFileSync(path.join(__dirname, 'mixins.pug'))
}