Add option to be non-interactive.

[gitlost]

See https://github.com/wp-cli/package-command/pull/60#pullrequestreview-85985229

Composer supports being run non-interactively so it could be useful to have an option to set this.

In particular it would avoid getting prompted for one's ssh password as can happen depending on one's authorization settings, which is problematic for scripts.

[schlessera]

Yes, agree, we can simply forward such an option. It's an easy addition.