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

[meinaart]

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.

[meinaart]

Can someone please take a look at this? Preferably @tschaub :)

[meinaart]

See the documentation here: https://github.com/meinaart/grunt-newer/tree/newer-option

[tschaub]

I appreciate the work you've put into this @meinaart. However, I'd like to avoid adding configuration options to other tasks.