Autocompletion

[felixSchl]

It should be fairly doable to match the current input and then upon success (or failure) use the longest match to offer choices to complete the next thing.

Problems:

• Deal with user typing in string. Or do we have to? In bash, typing "-- is the same as typing --.

• There may be more than one branch that qualifies. Show all relevant options.

• Need to provide hooks, so users can bring their own completion function for option arguments and arguments.

• We need to know "where" we are in a branch after completion stops:

  foo --bar QUX --foo --moo
  $ # -> [--bar]
  $ --bar # -> []
  $ --bar "qux" --foo # -> [--moo]
  $ --bar "qux" --foo --moo # -> []