Option to emit attribute to disable rustfmt

[danielparks]

Depending on how rustfmt is configured, it may try to reformat ucd-generate output. Add an option to emit #![rustfmt::skip] at the top of files created by ucd-generate so that rustfmt knows to skip them.

See rust-lang/regex#634 for an example where files generated by ucd-generate don’t match rustfmt’s configuration, which cause CI failures.

[BurntSushi]

I'm in favor of adding this, but this really isn't causing the CI failures. This is just kind of one of the hazards of putting a rustfmt check in CI. rustfmt evolves even if the code doesn't. Generated code is just as susceptible to this as regular code. Indeed, in regex's case, some hand-written files were also bitten by this.