ls: implement --color=auto behavior

The automatic behavior is to turn on colors only of connected to a
terminal.
Introduce new external crate isatty to handle this.

Signed-off-by: Gabriel Ganne <gabriel.ganne@gmail.com>

src/ls/Cargo.toml

@@ -10,6 +10,7 @@ path = "ls.rs"
 
 [dependencies]
 getopts = "0.2.18"
+isatty = "0.1"
 number_prefix = "0.2.8"
 term_grid = "0.1.5"
 termsize = "0.1.6"

src/ls/ls.rs

@@ -14,6 +14,8 @@ extern crate termsize;
 extern crate time;
 extern crate unicode_width;
 extern crate number_prefix;
+extern crate isatty;
+use isatty::stdout_isatty;
 use number_prefix::{Standalone, Prefixed, decimal_prefix};
 use term_grid::{Cell, Direction, Filling, Grid, GridOptions};
 use time::{strftime, Timespec};
@@ -615,7 +617,7 @@ fn display_file_name(
         None => true,
         Some(val) => match val.as_ref() {
             "always" | "yes" | "force" => true,
-            "auto" | "tty" | "if-tty" => true, /* TODO */
+            "auto" | "tty" | "if-tty" => stdout_isatty(),
             "never" | "no" | "none" | _ => false,
         },
     };