+ Generalize the CommandInfo type by parameterizing it on the monad
instead of just the environment.
+ Generalize the commands defined in
GF.Command.{Commands,Commands2,CommonCommands,SourceCommands,HelpCommand}
to work in any monad that supports the needed operations.
+ Liberate GF.Command.Interpreter from the IO monad.
Also, move the current PGF from CommandEnv to GFEnv in
GF.Interactive, making the command interpreter even more generic.
+ Use a state monad to maintain the state of the interpreter in
GF.{Interactive,Interactive2}.
Created module GF.Command.CommonCommands with ~250 lines of code for commands
that do not depend on the type of PGF in the environemnt, either because they
don't use the PGF or because they are just documented here and implemented
elsewhere.
TODO: further refactoring so that documentation and implementation of
*all* commands can be kept together.
