Suggestion: Provide shell completions

[CosmicHorrorDev]

Note: I'm open to opening a PR for this

Since samply is using clap you can use clap_complete to generate shell completions with relative ease

One question that will need to be resolved is how to provide completions. The two most common ways are through either at build time through a build script or at runtime through some CLI flag or subcommands. Any preference on which one should be used?