Create the screenpy-quickstart extension for ScreenPy Selenium.

[perrygoy]

ScreenPyHQ/screenpy#52

To resurrect the screenpy-quickstart script now that we have everything all split out, we'll need to make quickstart functions for each of the extensions. These functions will take in the directory in which to make the files, then create the files specific to that extension in a screenpy_extensionname subdirectory.

For ScreenPy Selenium, put all the files in screenpy_selenium, obviously!