Add basic completions for shallow-backup

[alichtman]

This adds basic tab completion for https://github.com/alichtman/shallow-backup/

This will fix https://github.com/alichtman/shallow-backup/issues/324.

I am the original author of this piece of software.

• [x] This compdef is not already available in zsh.
• [x] This compdef is not already available in their original project.
• [x] I am the original author, or I have authorization to submit this work.
• [x] This is a finished work.
• [x] It has a header containing authors, status and origin of the script.
• [x] It has a license header or I accept that it will be licensed under the terms of the Zsh license.

[alichtman]

Thanks! @syohex