config.yaml handling is necessary in Go port

[Gustavo Niemeyer]

The Go port must be able to handle formula config.yaml
files. In that first step, just parsing and processing
will be done. Following that, validation of variables
using the parsed schema should be implemented.

It won't support the 'str' type variant being obsoleted
for the moment. We can do a new evaluation before opening
up the formula and introduce it if necessary, depending
on the migration rate for the new 'string' value.