Improve subclassability: pass handle 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).
Improve subclass-ability: factor out some regex

...so that they can be overridden in subclasses.

For example, in Config::INI::Writer::HereDoc a newline is a valid
character in a value.
Make the filehandle an attribute of the reader.

Ricardo pointed out that it only passing the handle to one or
two routines was kind of odd, and passing it to all of them was
also a bit ugly.  His solution was to make it an attribute of the
reader.

This commit does that.
Don't return regexps, use affirmative predicate tests

Change routines that were returning regular expressions that client
code then needed to Do The Right Thing with into predicates that test
whether a thing is valid or not.  (Based on Ricardo's feedback).
revert regression to definition of handle_unparsed_line
Revert "Make the filehandle an attribute of the reader."

This reverts commit 1462ea5445b2f3dc4063f27f1312077e56c08331.
make handle-passing consistent