wrap examples

[Aratramba]

add the ability to wrap pug code around examples, for each example separately and globally:

//- @pugdoc
  name: Foo
  wrap: |
    div.wrap
  examples:
    - | span 1
    - | span 2

should output:

<div class="wrap"><span>1</span><span>2</span></div>

Question: What should the wrap option do for individual examples? Ignore? Or also wrap?
(Maybe a solution for the grouped-example could be to add a separator option?)

---

//- @pugdoc
  name: Foo
  wrapEach: |
    div.wrap
  examples:
    - | span 1
    - | span 2

should output:

<div class="wrap"><span>1</span></div><div class="wrap"><span>2</span></div>