Add support for helpcenter

This pull request is to add support for help center api.

### Introduction
Helpcenter feature can be enabled by -hc or --helpcenter or {helpcenter: true} in options.
When helpcenter is enabled, main site api will be disabled (`helpcenterParts` instead of `parts` will be exported in client.js).

### What have been done
1. All apis at https://developer.zendesk.com/rest_api/docs/help_center/introduction (except for "article attachment creation") have been implemented.  
2. Most of apis have been tested locally except for votes (I do not find this feature, not available for free user I guess)  
3. ReadMe have been updated for helpcenter api and -hc / --helpcenter argument.
4. client.js has been modified to be capable with helpcenter api (I'm not sure if I am doing the right thing...)


### Todo
1. Examples
2. Tests