Fixes #19 - Allow executing puppeteer commands from mocha tests.

[austinkelleher]

Implements a window.puppeteerCommand(...) function that can be used in the mocha tests.

describe('screenshot test', function () {
  it('should take a screenshot', async () => {
    await window.puppeteerCommand({
      type: 'screenshot', // The page command to run
      args: [ {           // The arguments that should be passed to the page command
        path: './test.png'
      } ]
    })
  })
})

[coveralls]

Coverage increased (+0.06%) to 98.841% when pulling 68ada77 on 19-commands into 3c7677d on master.