Allow task-specific Composer arguments and apply defaults

[arithmetric]

This fixes #319.

Changes:

• Initializes grunt-composer after configuration to enable multi task mode and task-specific configuration.
• Updates default tasks for multi task mode.

At this point, this has a potential breaking change, since the Grunt commands composer:install and composer:update no longer work and must be changed to composer:install:install and composer:update:update.

[grayside]

The doubling up of the task target looks really strange, is the idea that the first instance tells composer which command to run and the second which configuration to use?

[grayside]

This is surfacing bugs in the package command for Drupal 8, which we already know is badly broken. Going to play with this some more but will probably merge as a precursor to fixing package.