Add methods to Tester class for more flexibility

These methods are largely inverses of previous methods, and are not yet used
in the main program.  They are:
- consider_whitespace(), the opposite of ignore_whitespace()
- heed_char(), the opposite of ignore_char()
- heed_all_chars(), which removes all characters from the ignore list as well
  as considering all whitespace.
- expect_proper_length(), which undoes both ignore_extra_chars() and
  ignore_too_few_chars().