Update frankjpeg.py

[BobVonBob]

Renames sub-command "-" to "num".

Adds new sub-command "name" which chooses an image by name.

Clarifies usage in command descriptions.

Cleanup on list sub-command:

• Use enumerate() instead of a manual index counter.
• Use os.path.basename() instead of a slice to remove path to file.
• Remove underline from file name to avoid hiding underscores.