CodeMirror: YAML front matter mode

YAML front matter mode

---
receipt:     Oz-Ware Purchase Invoice
date:        2007-08-06
customer:
    given:   Dorothy
    family:  Gale

items:
    - part_no:   A4786
      descrip:   Water Bucket (Filled)
      price:     1.47
      quantity:  4

- part_no:   E1628
      descrip:   High Heeled "Ruby" Slippers
      size:       8
      price:     100.27
      quantity:  1

bill-to:  &id001
    street: |
            123 Tornado Alley
            Suite 16
    city:   East Centerville
    state:  KS

ship-to:  *id001

specialDelivery:  >
    Follow the Yellow Brick
    Road to the Emerald City.
    Pay no attention to the
    man behind the curtain.
---

GitHub Flavored Markdown
========================

Everything from markdown plus GFM features:

## URL autolinking

Underscores_are_allowed_between_words.

## Strikethrough text

GFM adds syntax to strikethrough text, which is missing from standard Markdown.

~~Mistaken text.~~
~~**works with other formatting**~~

~~spans across
lines~~

## Fenced code blocks (and syntax highlighting)

```javascript
for (var i = 0; i < items.length; i++) {
    console.log(items[i], i); // log them
}
```

## Task Lists

- [ ] Incomplete task list item
- [x] **Completed** task list item

## A bit of GitHub spice

* SHA: be6a8cc1c1ecfe9489fb51e4869af15a13fc2cd2
* User@SHA ref: mojombo@be6a8cc1c1ecfe9489fb51e4869af15a13fc2cd2
* User/Project@SHA: mojombo/god@be6a8cc1c1ecfe9489fb51e4869af15a13fc2cd2
* \#Num: #1
* User/#Num: mojombo#1
* User/Project#Num: mojombo/god#1

See http://github.github.com/github-flavored-markdown/.

Defines a mode that parses a YAML frontmatter at the start of a file, switching to a base mode at the end of that. Takes a mode configuration option base to configure the base mode, which defaults to "gfm".