Make clap dependency optional

[svix-jplatte]

As far as I can tell, both the clap and byte-unit dependencies are only there to allow the Config type to be parsed from command-line arguments. I would send a simple PR, but due to the impl Default for Config it's not entirely trivial.

Would you be open to a PR that makes clap and byte-unit optional and either

• duplicates the default values from the clap attributes into the Default implementation or
• adds a custom macro for the Default trait that takes the clap attributes into account or
• removes support for parsing Config from CLI args entirely (it doesn't appear to be used in this repo)?

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