CLI: SB3 output

[towerofnix]

Following #129, we should be able to export to the other formats sb-edit has complete or partial support for.

Some notes:

• Infer output type from output file extension .sb3, or -ot sb3 / --output-type sb3, as usual
• SB3 conversion (Implement SB3 output #33) is a normal, complete roundtrip: you input a whole project and get out a whole project, like converting SB3 to Leopard
• Overwriting the input SB3 would always probably be a bad idea, but the CLI already has examples of avoiding overwriting. We may want to generally implement a warning like, "Output file must not be input file." rather than the current message "Output file already exists."