New Driver class which inherits from parent (#99)
New Driver class which inherits from parent (#99)

* `main` and `run` (which is the main `svgtiler` function) now run
  within a `Driver` class that keeps track of that driver's settings.
* `getDriver` and `runWithDriver` keep track of currently running
  Driver, and enable new Driver to inherit settings from parent.
* Array of `formats` pushed into settings for proper inheritance.
* `getSettings` now returns `defaultSettings` instead of `null`.

edemaine/svgtiler