Abstract templating language config

[husseinalhammad]

Both juice and email-comb can be configured so they are aware of templating language syntax.

Right now the plugins config handle each package's config separately (and hence templating language syntax has to be listed twice).

mix.email({
  source: 'dist/**/*.html',

  inlineCss: {
    enabled: true,

    // Automattic/juice config
  },

  optimize: {
    enabled: true,

    // codsen/email-comb config
  }
})

It would be nice to make this configurable through a single templating object:

mix.email({
  source: 'dist/**/*.html',

  templating: {
    // configure once, let the plugin pass the config to juice and email-comb
  },

  inlineCss: {
    enabled: true,

    // Automattic/juice config
  },

  optimize: {
    enabled: true,

    // codsen/email-comb config
  }
})