two project configuration features

[scolsen]

1. Add a "list" function to retrieve all known configuration fields in xobj form (we already have :p for a printed format)

   ;; not sure what the best name is
   (Project.config-keys)
   => ["cflags", "libflags", "compiler" ... etc.] ;; maybe make these tuples of (key, expected-value-type)?

2. Allow users to add new fields to the Project configuration field set -- it doesn't do anything special, just allows people to add certain information to the Project namespace to use it as a conventional place to stow configuration/build related values.