Add color output to zfs diff.

This adds support to color zfs diff (in the style of git diff)
conditional on the ZFS_COLOR environment variable.

Signed-off-by: Ethan Coe-Renner <coerenner1@llnl.gov>

man/man8/zfs.8

@@ -704,6 +704,12 @@ command will be undone if the share is ever unshared (like via a reboot).
 .El
 .
 .Sh ENVIRONMENT VARIABLES
+.Bl -tag -width "ZFS_COLOR"
+.It Sy ZFS_COLOR
+Use ANSI color in
+.Nm zfs Cm diff
+output.
+.El
 .Bl -tag -width "ZFS_MOUNT_HELPER"
 .It Sy ZFS_MOUNT_HELPER
 Cause