Make --color option behave more like BSD/GNU grep's --color option

Since MNG-7080 introduced this option we mimic grep's behavior, but not the way grep actually does.

GNU grep: https://git.savannah.gnu.org/cgit/grep.git/tree/src/grep.c#n2751
FreeBSD grep: https://github.com/freebsd/freebsd-src/blob/781c3b51193e5bd99773439192ca68fdfdd9c330/usr.bin/grep/grep.c#L576-L596

• Both make the value for --color optional and default to auto
• They also accept tty, if-tty, yes, force, none, no

We should support that approach too: Optional value and privately accepting additional values.