add middleware option

[Aratramba]

if we add an option to alter the components data feed before rendering components and pages, we could for example concat multiple components that share the same name, or decide not to on a component basis.

DesignManual.build({
  output: 'docs/',
  pages: './',
  components: 'docs/components.json',
  middleware: (components) => {
      // do something
      return components;
  },
  meta: {
    domain: 'example.com',
    title: 'Example'
  }
});

This solves a problem where collect components outputs multiple components with the same name - something design manual by design can not use.