Add support for Github-powered templates

New `template` option added to `harp init`:

```sh
$ harp init -h

  Usage: init initializes a new Harp project in the current directory.
  See available templates at https://github.com/harp-templates

  Options:

    -h, --help                             output usage information
    -t, --template <github-username/repo>  use a github repo as a template
```

Leave it blank and you get the default:

```sh
$ harp init myapp    
Downloading template: https://github.com/harp-templates/default
Initialized project at /Users/zeke/Desktop/myapp
```

Use a shorthand template name to default to the harp-templates github repo:

```sh
$ harp init my-stylus-app -t jade-stylus-markdown
Downloading template: https://github.com/harp-templates/jade-stylus-markdown
Initialized project at /Users/zeke/Desktop/my-stylus-app
```

Use any github–username/repo:

```
$ harp init another-app -t zeke/harp-template-thirdparty-example
Downloading template: https://github.com/zeke/harp-template-thirdparty-example
Initialized project at /Users/zeke/Desktop/another-app
```

I created a public org and added @sintaxi, @silentrob, and @kennethormandy as owners: https://github.com/orgs/harp-templates/members