Add custom discovery and base urls through args

You can now pass in a custom Discovery URL:

``` sh
node scripts/generate --discovery-url "http://discoveryurl.example.com"
```

Or a custom Base URL:

``` sh
node scripts/generate --base-url "http://baseurl.example.com"
```

**Note:** Adds devDependency: https://github.com/substack/minimist

Fixes #264.