[feat] Support additional characters in keys

[dcramer]

In our particular scenario we use a : to denote a separation of concern in features. In other cases we also use a . notation.

e.g. feature=organization:create or feature=auth.register

It'd be nice if Flagr could support these same kind of naming conventions, as I'm guessing there's no actual reason not prevent those two characters in key names currently.

[zhouzhuojie]

I think : and . are fine, and we recently added / to the characters set. The only concern is that if flagKey is going to be used in the URL path in the future, we may need to do some URL % encoding.