Use chalk module to colorize terminal output

- Add [chalk](https://github.com/sindresorhus/chalk) as a dependency.
- Use chalk to colorize terminal output and get rid of terminal ANSI escape codes.
- Minor changes in message strings (see https://github.com/radkodinev/grunt-contrib-uglify/commit/9a17225e478d96515f872bfa2e9d73dd8f8da119#diff-0c8f71b28b3ffffd4d019110b7446b88R138 for source map message).