Pull out initial less recipe.

* factor out a module for declarative building of gulp less tasks
* generalize gib configuration for less.

This takes the separates the declarative gibberish configuration from
how to build and configure the actual gulp tasks.  The less recipe is
usable and re-usable by itself.  The gulpfile now parses our gib config
and maps it to the recipe.  Next up is generalizing that.