Default and custom Composer arguments not applied

[arithmetric]

Currently, the default Composer arguments for install and update commands are not applied to the Composer command at run-time.

This can be verified by altering task/composer.coffee in grunt-composer to add debugging output before the command is exec'ed. See: https://gist.github.com/arithmetric/9aa44be88d8bde383a8bbe0d7a0e1b2b

Additionally, it is not possible to re-configure the Composer arguments for a particular Composer task (i.e. only for install and update, not for drupal-scaffold).

This is due to grunt-composer being initialized in "simple task mode" instead of "multi task mode". See: https://www.npmjs.com/package/grunt-composer#simple-task-mode