* Support a "is_global" and "is_private" member on Rules.

When writing linters it is currently impossible to know (via rule introspection)
if a given rule is private or global. We have banned global rules for our use
case and we have to resort to a janky regex against our rules files to know if
anyone is about to commit a global rule. I figure exposing these two flags via
python will be useful for programatically checking those bits.

I'm not very pleased with the name "is_global" - I wanted to go with just
"global" and "private" but "global" is a reserved keyword and rule.global breaks
the python interpreter. I'm open to changing the member names if you have any
suggestions.

* Decrement reference counts on global and private.

* Update global and private checks after API changes.