Account for a custom-hosts file to hold project level hosts
Grab defined hosts from a custom-hosts file and make the VM aware

This allows arbitrary custom domains to be defined on a project basis and eliminates the need to maintain a huge list of dev domains in Vagrantfile
Only specify one default domain - wp.wsu.edu

Others should be customized through custom-hosts
Cleanup documentation around custom hosts a bit.
Merge pull request #13 from washingtonstateuniversity/13-hosts

Provide a method for adding custom hosts