Use global variable to combine plugin results with results of require-css plugin

This is the second attempt of PR https://github.com/guybedford/require-less/pull/72

Global variable is used to share css content between plugins. This variable should be deleted after each build to prevent content accumulation between several consequences build runs. Simple reference handling (`usedBy` property) is used to understand when all the plugins finish their work.

This is not very elegant solution, but looks like it works. I've made the following tests:
  * requirejs optimization of the whole project
  * requirejs optimization of a single file
  * two consequence runs of optimization task using Grunt

I'll create corresponding PR to `require-css` plugin If this solution is ok.
I think you may want to put this code in `write` instead of `load` as `write` will be called after each layer, while `load` is called during the initial tracing only.