Added "force" option to allow to continue running Grunt in the event of failures

[andrewholgate]

• Added "force" option to allow to continue running Grunt in the event of failures.
• Removed empty spaces.

[grayside]

1. Sending the error object to done() is potentially useful for troubleshooting. Maybe it could be conditionally set? E.g. options.force ? done() : done(error);
2. In 0.3.0 there was an ignoreExitCode, for backwards compatibility it would be great to support that.

[grayside]

Recently came across https://www.npmjs.com/package/grunt-force-task, which might prove a general workaround that keeps grunt-phpcs slightly simpler.

[gwagroves]

@grayside Might be a good idea to mention the grunt-force-task workaround in the README.