less Parser in less-builder uses requirejs config.less.relativeUrls

I ran into a case where I could manually `lessc --relative-urls` successfully, but setting my requirejs `config.less.relativeUrls = true` would result in a failed build.

Currently when optimizing a file using require-less, the Less parser is not passed any of the options in `config.less`. They are only passed to the `toCSS` method on the parsed tree.

However, there are times when a user needs their config options passed to the Parser. This PR currently only does that for `relativeUrls`, but I'm eager for suggestions on a more general solution.

What do you think?