text: Title: Clarify which methods allow keyword arguments or require a dict
  Body: In the README it [says][1]:
  
  > puppeteer uses an object for passing options to functions/methods. pyppeteer methods/functions accept both dictionary (python equivalent to JavaScript's objects) and keyword arguments for options.
  > 
  > ### Dictionary style options (similar to puppeteer):
  > 
  > ```
  > browser = await launch({'headless': True})
  > ```
  > 
  > ### Keyword argument style options (more pythonic, isn't it?):
  >
  > ```
  > browser = await launch(headless=True)
  > ```
  
  This is great and I 100% prefer the keyword argument-style options. But not all methods seem to support them, for example [`page.setViewport`][2]:
  
  ```
  await page.setViewport(width=800, height=600, deviceScaleFactor=2)
  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  <ipython-input-50-4d077b87a1ec> in <module>
  ----> 1 await page.setViewport(width=800, height=600, deviceScaleFactor=2)
  
  TypeError: setViewport() got an unexpected keyword argument 'width'
  ```
  
  I think it would be best if all the methods in the pyppeteer API supported keyword arguments but, barring that, documentation on which ones do would be second best.
  
  Thanks for the great library, it's exactly what I was looking for!
  
  [1]: https://github.com/pyppeteer/pyppeteer#keyword-arguments-for-options
  [2]: https://pyppeteer.github.io/pyppeteer/reference.html#pyppeteer.page.Page.setViewport