Add feature: gnu_legacy mode for ansi styles

[alexkunde]

Hi,
gnu coreutils are still using double digit styles instead of single digit styles.
To make this available in uucoreutils (the rust drop in replacement for gnu core utils) I added a feature called gnu_legacy.
Related PR

By turning it on it will generate styles double digit. If not turned on it will behave as before. Tests have been added.

// without feature flag gnu_legacy
    test!(bunderline:            Style::new().bold().underline();   "hi" => ,"\x1B[1;4mhi\x1B[0m");
// with feature flag gnu_legacy
    test!(bunderline:            Style::new().bold().underline();   "hi" => "\x1B[01;04mhi\x1B[0m");

Please see the code and I hope this feature can make it.

The second missing feature would be to have the option to reset a style before applying. I would open another PR for this.

[fdncred]

looks like some tests are failing

[alexkunde]

Hi, locally it correctly tests now. I had to add some wrapper around the docs. Now the feature tests are documented on functions as well. All lines starting with # are evaluated but not shown as a tooltip.

[fdncred]

Thanks