Got fig paths renumbered for aastex

Introduced two options to the pack command:

1. `--style` can be set to 'aastex' to show that we want to make a
   AAS-journal compatible build. This triggers renumbering figures as f1, etc
2. `--exts` can be used to set the priority of what what figure file
   format to prefer when copying figures into the build folder.
Fix how figures are renamed in tex

Rather than use the re module I just do a string replacement.

Note there's some kludgyness in that the figure environment string is
stored with an escaped slash, whereas the options strings aren't. This
should be standardized.
Figure numbers start at 1
aastex build style writes ms.tex now