Compile Procfile & nginx.conf from gdk.yml if possible

This is a first step towards to #413 (closed).

With the proposal in that issue, we want to make configuration management more centralized. Since I considered it too big to convert everything to rake at once, I got started with Procfile.

Details

All the magic happens in lib/gdk/config.rb. This file defines the structure of the config file, and sets the defaults. Settings can be overridden by adding a gdk.yml at the root of the project, or using the already used lose files like hostname, port, webpack_port, etc.

The Procfile is generated from Procfile.erb, using rake Procfile. From here on, also other config files can be generated through ruby.

So far the use of a gdk.yml is experimental, and also undocumented. It will need a few other MRs before it becomes usable by anyone. But the lose files still work, so this MR should not get in the way of anyone.