Add support for filtered/public configuration

I've included some samples in the tests dir.  This goes against the grain with what I've seen in with the rest of the tests, where test fixtures are generated and destroyed.  IMO it is much easier to understand how configurator works when there are tangible examples.

As for the implementation, I decided to go with a file called `public-data.json` in app config.  This information is totally different and unlike the other config so I thought it appropriate if the file unlike the others.

ping @stefanor

closes https://github.com/yola/yoconfigurator/issues/28


I think so.  My reasoning - looking [here in configurator](https://github.com/yola/yoconfigurator/blob/7bb1009a5102e71ee786614e517c52408474e13c/bin/configurator.py#L84-L95) we have two distinct units of work:

1. create configs
1. output configs

The multiple calls would exist under `if not options.dry_run:` leaving me two two ugly options: a loop nested under a conditional or two calls to write (not very dry).

I'm open to suggestion.