Added support for grunt-newer to plugins that do not adhere to Grunt standards

Added support for grunt-newer to plugins that do not adhere to Grunt standards, best example of this is grunt-spritesmith. The alternative grunt-spritely is not actively maintained and does not support all grunt-spritesmith's features.

Can also be used for other plugins that have a custom output declaration scheme.

Documentation, package.json and tests are updated to reflect my changes.