Added JPEG support. Fixes #4

This PR builds on top of #6 and adds JPEG support to `save-pixels`. For an interim diff, view:

https://github.com/twolfson/save-pixels/commit/519ab43f24013b56da003f7ed5dac5fb266ff312

In this PR:

- Broke down test utilities and added test for JPEGs that assert against expected images
    - There was loss during `save-pixels` due to being a JPEG
- Added case to support `JPG`, `.JPG`, `JPEG`, and `.JPEG` image types