README: Promote "Configuration" section to an <h2>
README: Promote "Configuration" section to an <h2>

This moves it out from under the "Installation" section.
* The "Installation" section is already pretty long
* The line under `<h2>`s in GitHub Markdown helps clarify that it's not a subsection of the "Bower" or "Node.js Mincer" sections
* Installation and configuration are arguably separate topics

SolarCS/bootstrap-sass,SolarCS/bootstrap-sass,SolarCS/bootstrap-sass,SolarCS/bootstrap-sass