Package type common-webroot

Proposal for webroot themed packages.  In #38 drupal-core places the package in core.  This commit exposes a new extra in the inflected vars `{$webroot}` that can be used by packages.  It forces `installer-webroot` to be defined on the root package for a `common-webroot` package.

This proposes a package to be defined as a webroot application with a variable webroot location.  A way to assert that there is only one package of that type would be ideal, not quite sure how to implement that.