use 'atty' to detect when colors can be used

Cargo.lock

@@ -2264,6 +2264,7 @@ name = "taskchampion-cli"
 version = "0.2.0"
 dependencies = [
  "assert_cmd",
+ "atty",
  "config",
  "dirs 3.0.1",
  "env_logger",

cli/Cargo.toml

@@ -13,6 +13,7 @@ nom = "*"
 prettytable-rs = "^0.8.0"
 textwrap = "0.12.1"
 termcolor = "1.1.2"
+atty = "0.2.14"
 
 [dependencies.config]
 default-features = false

cli/src/invocation/mod.rs

@@ -18,7 +18,7 @@ pub(crate) fn invoke(command: Command, settings: Config) -> Fallible<()> {
     log::debug!("command: {:?}", command);
     log::debug!("settings: {:?}", settings);
 
-    let mut w = StandardStream::stdout(ColorChoice::Auto);
+    let mut w = get_writer()?;
 
     // This function examines the command and breaks out the necessary bits to call one of the
     // `execute` functions in a submodule of `cmd`.
@@ -110,3 +110,12 @@ fn get_server(settings: &Config) -> Fallible<Box<dyn server::Server>> {
         client_id,
     })?)
 }
+
+/// Get a WriteColor implementation based on whether the output is a tty.
+fn get_writer() -> Fallible<StandardStream> {
+    Ok(StandardStream::stdout(if atty::is(atty::Stream::Stdout) {
+        ColorChoice::Auto
+    } else {
+        ColorChoice::Never
+    }))
+}