Allow emojis to be force enabled with --emoji.

The previous behaviour had a default of enabling emojis for Macs, and
allowed explicitly disabling them.

This PR keeps that default but allows `--emoji` to enable for other
platforms if the user wants to.  `--no-emoji` still works as before, but
due to the way that commander processes "--no-" flags (forcing the
default to true and overwriting the default value given in the `.option`
definition) it was necessary to switch the flag definition from
`--no-emoji` to `--emoji`.

src/cli/index.js

@@ -65,8 +65,9 @@ commander.option(
   'use a mutex to ensure only one yarn instance is executing',
 );
 commander.option(
-  '--no-emoji',
-  'disable emoji in output',
+  '--emoji',
+  'enable emoji in output',
+  process.platform === 'darwin',
 );
 commander.option(
   '-s, --silent',
@@ -131,7 +132,7 @@ commander.args.shift();
 //
 const Reporter = commander.json ? JSONReporter : ConsoleReporter;
 const reporter = new Reporter({
-  emoji: commander.emoji && process.stdout.isTTY && process.platform === 'darwin',
+  emoji: commander.emoji && process.stdout.isTTY,
   verbose: commander.verbose,
   noProgress: !commander.progress,
   isSilent: commander.silent,