Allow colour to be configured with DFT_COLOR too

In #468 it's also mentioned that this couldn't be configured with an
environment variable.

CHANGELOG.md

@@ -5,6 +5,9 @@
 `--help` now shows the default value for all arguments (it was
 previously missing `--color` and `--display`).
 
+`--color` can now be configured with the environment variable
+`DFT_COLOR`.
+
 ## 0.42 (released 15th January 2022)
 
 ### Parsing

src/options.rs

@@ -154,6 +154,7 @@ fn app() -> clap::Command<'static> {
             Arg::new("color").long("color")
                 .possible_values(["always", "auto", "never"])
                 .default_value("auto")
+                .env("DFT_COLOR")
                 .value_name("WHEN")
                 .help("When to use color output.")
         )