allow customizing more vagrantfile options
vagrant: allow customizing multi-arg settings
Extend vagrant *config* and *chef* customization support
 - Can customize "multi-dot" settings (e.g. "config.a.b.c")
 - Add customization example for proxy settings
 - Change the API for advanced settings customization
Add a note about vagrant settings customizations
to the contributing docs
Move customization notes to bottom of the list
in order to keep the steps of the typical situation clean and simple
CONTRIBUTING.md - formatting enhancement
Merge pull request #283 from dbfit/allow-customizing-more-vagrantfile-options

Allow customizing more Vagrant options