Add YAML config (`--config`) for shorter command

[mochi-yu]

Background

I run deep-copy from //go:generate. The whole command has to be one line—I cannot break it across lines. As I add more types and flags, that line gets very long. My diffs get noisy and I run into merge conflicts more often.

Idea

I want a YAML config file with the same options I already pass as flags.

• I would pass it with --config path/to/file.yaml.
• If I also set a flag on the command line, the flag should win over the file.

Then I can keep go:generate short, e.g. deep-copy --config deep-copy.yaml ., and put real changes in the YAML file.