* TYPO in test comment.  "check write_string()" -> "check
  read_string()"
* TYPO (?)  Increase clarity by adding missing label to next
  statement, matches the surround code style better.
* Tests first.  Add a test file and example data for the here-doc
  reader.
* Improve subclass-ability.  In read_handle(), pass the file handle
  into the call to parse_value_assignment.  The stock
  parse_value_assignment ignores it but fancier readers can use it to
  read multi-line values (e.g. here-doc style).
* touch up here-doc reader test, one of the tests was still
  calling read_string on Config::INI::Reader (left over from
  cut-and-paste from my previous version).
* Config::INI::Reader::HereDoc, a Config::INI::Reader subclass
  that supports heredoc style assignments.
* Improve subclass-ability.  Factor out several regular expressions
  from validate_input so that they can be overridden in subclasses.

  For example, in Config::INI::Writer::HereDoc a newline is a valid
  character in a value.
* a test for the heredoc-enabled writer.
  * Config::INI::Writer::HereDoc, a Config::INI::Writer subclass
    that supports values with embedded newlines using heredoc style
    assignments.