Add --dump-default-config and --dump-minimal-config.

Add --dump-default-config and --dump-minimal-config.

 - `--dump-default-config` outputs the default configuration to the
   specified file as TOML and then exits.
 - `--dump-minimal-config` is checked after formatting files as normal.
   If present, any configuration options that were checked during
   formatting are written to the specified file as TOML.
 - These options were added only to `rustfmt`, not to `cargo fmt`. They
   can be specified when using `cargo fmt` by placing them after `--`.
 - It would have been nice if the filename was optional, so you could
   run just `rusfmt --dump-minimal-config build.rs` to have it output to
   `rustfmt.toml`. However, this doesn't do what you might expect: it
   outputs the config to `build.rs`!