Should only enable colors if there's a terminal attached

[OnkelTem]

Consider an example:

import 'package:chalkdart/chalk.dart';

void main() {
  print(chalk.blue('Hello World!'));
}

When we run it with dart run, it outputs blue text in the terminal.

However if we redirect the output to a file with dart run > out.txt, the escape-sequences are also get copied:

�[34mHello World!�[39m

I believe it's not a correct default behavior. In the simplest case, we can detect the presence of the terminal this way:

import 'dart:io';
import 'package:chalkdart/chalk.dart';

void main(List<String> arguments) {
  if (stdout.hasTerminal) {
    print(chalk.blue('Hello World!'));
  } else {
    print('Hello World!');
  }
}

— but at the chalkdart's side.

[OnkelTem]

I found a better way to force chalkdart to disable colors:

import 'dart:io';
import 'package:chalkdart/chalk.dart';

void main() {
  Chalk.ansiColorLevelForNewInstances = stdout.hasTerminal ? 3 : 0;
  print(chalk.blue('Hello World!'));
}

Still I believe it should be the default chakdart behavior to allow for clean piping to files.