CodeMirror: GFM mode

GFM mode

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

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

(Set `gitHubSpice: false` in mode options to disable):

* 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

(Set `emoji: false` in mode options to disable):

* emoji: :smile:

Optionally depends on other modes for properly highlighted code blocks.

Gfm mode supports these options (apart those from base Markdown mode):

gitHubSpice: boolean

Hashes, issues... (default: true).
taskLists: boolean

- [ ] syntax (default: true).
strikethrough: boolean

~~foo~~ syntax (default: true).
emoji: boolean

:emoji: syntax (default: true).

Parsing/Highlighting Tests: normal, verbose.