Enable JPEG image conversion for preprint pack

If the preprint pack --style arxiv option is chosen, then any large
PDF/EPS figures will be converted into JPEGs as they are copied to the
build directory. This helps us stay within arXiv's upload size limit.

Changes:

- Require explicit choices for pack --style option
- Add pack --jpeg and --maxsize options
- Get figure file sizes when looking for figures
- Call imagemagick's `convert` to compress large figures.

Fix #2