#37 add support for variables

[easingthemes]

Description

Add variables support in runQuery mehod.

   /*
   * @param {string|object} body - the query string **or an object with query (and optionally variables) as a property**
   */
  async runQuery (body, options = {}, retryOptions = {})

Related Issue

Fixes #37

Motivation and Context

How Has This Been Tested?

Unit tests

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.