Add `saveScreenshot` API which actually saves to the filesystem.

The `takeScreenshot` API just returns base64 encoded data of the screenshot.
This method is more user-friendly and actually saves the image to the filesystem.

Usage:

browser
  .elementById("login")
  .click()
  .saveScreenshot("Logged_In")
  .elementById("logout" )
  .click()
  .saveScreenshot("Logged_Out");