+ 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}.
These commands are now implemented as regular commands (i.e. using the
CommandInfo data type) in the new module GF.Command.SourceCommands.
The list of commands exported from GF.Command.Commmands now called pgfCommands
instead of allCommands.
The list allCommands of all commands is now assembled
from sourceCommands, pgfCommands, commonCommands and helpCommand in
GF.Interactive.
