Support for rules files, and specifically for the "skip" rule:

IRSpy object now has an array of 0 or more rules.
New method apply_rules() to parse rules for later application [misnamed].
IRSpy object now has a set of name=>value variables.
New method var() for setting and getting variables.
New method should_skip_test() consults rules for a named test.
Skip a named test if a rule says to do so.
Some fiddling with split() calls to avoid confusing Emacs's Perl mode.
Add die() calls to the methods _last_sibling_test() and
	_next_sibling_test() which seem never to be called.  Assuming
	they don't start firing, I will remove these methods soon.