Support for colors. Using green, which might be invisible to some!

Probably those who can't see red or green will have configured their
terminal to display different colors instead.

Cargo.toml

@@ -11,6 +11,8 @@ failure-tools = "4.0.2"
 structopt = "0.2.14"
 jwalk = "0.4.0"
 byte-unit = "2.1.0"
+termion = "1.5.2"
+atty = "0.2.11"
 
 [[bin]]
 name="dua"

src/aggregate.rs

@@ -83,10 +83,13 @@ fn write_path(
     num_bytes: u64,
     num_errors: u64,
 ) -> Result<(), io::Error> {
+    use termion::color;
     writeln!(
         out,
-        "{:>10}\t{}{}",
+        "{}{:>10}{}\t{}{}",
+        options.color.display(color::Fg(color::Green)),
         options.format_bytes(num_bytes),
+        options.color.display(color::Fg(color::Reset)),
         path.as_ref().display(),
         if num_errors == 0 {
             Cow::Borrowed("")

src/common.rs

@@ -1,4 +1,5 @@
 use jwalk::WalkDir;
+use std::fmt;
 use std::path::Path;
 
 pub enum ByteFormat {
@@ -12,9 +13,39 @@ pub enum Sorting {
     Alphabetical,
 }
 
+#[derive(Clone, Copy)]
+pub enum Color {
+    None,
+    Terminal,
+}
+
+pub struct DisplayColor<C> {
+    kind: Color,
+    color: C,
+}
+
+impl Color {
+    pub fn display<C>(&self, color: C) -> DisplayColor<C> {
+        DisplayColor { kind: *self, color }
+    }
+}
+
+impl<C> fmt::Display for DisplayColor<C>
+where
+    C: fmt::Display,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        match self.kind {
+            Color::None => Ok(()),
+            Color::Terminal => self.color.fmt(f),
+        }
+    }
+}
+
 pub struct WalkOptions {
     pub threads: usize,
     pub format: ByteFormat,
+    pub color: Color,
 }
 
 impl WalkOptions {

src/main.rs

@@ -4,7 +4,7 @@ extern crate structopt;
 
 use structopt::StructOpt;
 
-use dua::ByteFormat;
+use dua::{ByteFormat, Color};
 use failure::Error;
 use failure_tools::ok_or_exit;
 use std::{io, io::Write, path::PathBuf, process};
@@ -20,6 +20,11 @@ fn run() -> Result<(), Error> {
     let walk_options = dua::WalkOptions {
         threads: opt.threads.unwrap_or(0),
         format: opt.format.map(Into::into).unwrap_or(ByteFormat::Metric),
+        color: if atty::is(atty::Stream::Stdout) {
+            Color::Terminal
+        } else {
+            Color::None
+        },
     };
     let (show_statistics, res) = match opt.command {
         Some(Aggregate {