add ICommand for new :ex-style commands

add ICommand for new :ex-style commands

These are needed for the new summary tables feature
(and inspired by the Command lookup table in init.c)

added: ICommand wrapper / lookup table allowing easy addition of new
informational commands that will not be parsed in .muttrc files and
override any existing rc-line commands when executed in mutt.

added: hook in mutt_enter_command in command.c to integrate new
ICommands facility

Issue #162