Provide new header liboac/util/enum.h

This new header is aimed to provide utility code to handle enum types.
The first utility is a set of template functions and a macro that
allow easy and short declaration of conversions between enum types
and string.
Implement config reading from file

As reported in #52, FlightVars requires the ability to read its
configuration from a file. This change adds the code necessary
to implement such a feature.