Per-repo default parameters (chunker, compression)

I think usability of borg could be improved by being able to set default values for certain options at repo initialisation time.

For example: `borg init --chunker-params-default 18,23,21,4096 -compression-default lz4 my-vm-image-repo`

Per the above example, any subsequent `borg create` invocation would use the chunker params and compression defaults specified above, but could be overridden by explicitly specifying the standard `--chunker-params` and/or `-C` arguments following `borg create`

Creating per-repo defaults for chunker params and compression would scratch my immediate itch, but on further thought there are many other post-init invocation arguments for which per-repo defaults may improve usability, such as `umask`. `no-files-cache`, etc.

The most obvious ways to implement this feature would be either:
1. Along the lines of my example above - that is, a slew of `--XXXXX-default` optional arguments to the `borg init` command.
2. A new borg command such as `borg repo-set-defaults --chunker-params 18,23,21,4096 --compression lz4` etc that could be invoked (if desired) after `borg init`

Cheers