Add ability to use querystring for individual flags

[jamesrwilliams]

You can only currently preview groups via the query string feature. I suspect it would be super useful to have this feature also applied to the individual flags. Similar to how groups use the group parameter flags should use flags or some other user-configurable string.

• Support custom flag query string values rather than the key.
• Probably should be opt-in rather than on by default.