Store config as YAML rather than JSON

Config should be written for humans, not computers. JSON can be daunting for non-coders, or even non-JS developers, and a single missing comma can break all the things.

YAML is much friendlier on the eyes, and much easier to edit for the uninitiated, lowering the barrier to entry for using Jekyll-hook. Example:

## Old Busted

```json
{
    "gh_server": "github.com",
    "temp": "/home/ubuntu/jekyll-hook",
    "scripts": {
        "build": "./scripts/build.sh",
        "publish": "./scripts/publish.sh"
    },
    "email": {
        "user": "", 
        "password": "", 
        "host": "", 
        "ssl": true
    },
    "accounts": [
        "developmentseed",
        "mapbox"
    ]
}
```

## New Hotness

```yaml
gh_server: github.com
temp: /home/ubuntu/jekyll-hook
scripts:
  build: ./scripts/build.sh
  publish: ./scripts/publish.sh
email:
  user: ""
  password: ""
  host: ""
accounts:
  - developmentseed
  - mapbox
```

Understand there's a bit of personal preference here, and don't want to start a holy war, so no hard feelings if you think it's best the project stays with JSON. :smile: