Integrate/feature dist

Here's the archiver integration into satis!

## Usage:

Add the following to your `satis.json`:

```json
{
    "archive": {
        "directory": "dist",
        "format": "tar",
        "prefix-url": "s3://my-bucket.s3.amazonaws.com",
        "skip-dev": true
    }
}
```

### Options

 * directory: relative to the current working directory, it's where the dist files are created
 * format: optional, the format (tar or zip) you want to create (default: zip)
 * prefix-url: optional, this is used in `packages.json` to prefix the download URLs (e.g. a private S3 bucket), otherwise the satis url is used
 * skip-dev: optional, do not create archives of branches (default: false)
  
I'll send a seperate PR to `composer/composer` to include this once this is done.