Added the Grunt config merge function

I added a simple method, `grunt.config.merge` (and alias, `grunt.mergeConfig`).
It behaves exactly as `grunt.initConfig`, but it allows the config object to be split into separate parts, or even separate files.

I've been using this to organize my grunt configurations by feature.  For example, I have sections like "desktop-javascripts", "mobile-javascripts", and "html templates".  Even though these features require overlapping tasks, the merge function allows them to be separated into more logical groupings.