Add KIF_SCREENSHOTS_DATE_FORMAT format filename of resulting image

1) Added code that would attempt to create a folder if KIF_SCREENSHOTS path does not exist
2) Added default filename format: HH-mm-ss - step.description
3) Added KIF_SCREENSHOTS_DATE_FORMAT environment variable that would replace the default "HH-mm-ss" portion of the filename
4) Updated README.md to reflect changes made
Merge pull request #2 from Evrst/bw-644

Add KIF_SCREENSHOTS_DATE_FORMAT format filename of resulting image