* support for arguments to flag options (though I want to test this more
  thoroughly)
* initial support for list_locals, which is of limited use because
  substitutions run as a function call with their own (empty) set of locals
* updating documentation to what the shell will be after the next few things
  that i will be fixing
* pulling out the repeated pattern of local_or_append_word and
  set_or_append_word, and some related variable_map refactoring