Optionally save png files

Adds a new config arg save_file_format which can optionally be png to save images as png format instead of the default jpg. png files have less compression so the text annotations are easier to read. Credit @donnlee for originally creating the PR for this.

Note that the optional arg always_save_latest_jpg is now renamed to always_save_latest_file since this can be either a jpg or png now

Ref #99