Patch updated:
- _UNSET removed from docs in favor of a more natural syntax (square brackets around optional arguments)
- _COMPATIBLE left in the docs because I find less magical in that case (+ it's already covered in the docs what this special case means)
- fixed docs improperly specifying `comment_prefixes` for ConfigParser and SafeConfigParser
- docs include a remark on where to find explanation for 'vars' and 'default' in every coercing getter
- tortured code refactored
- _unify_boolean renamed to _convert_to_boolean
- _UNSET and _COMPATIBLE moved to module level (test updates)
- _boolean_states renamed to BOOLEAN_STATES (for consistency with regular expression constants)
- _convert_to_boolean and BOOLEAN_STATES left in RawConfigParser class to enable users customize them (e.g. by specifying locale-specific 'yes'/'no' pairs or things like 'enable'/'disable', etc. etc.). Customization should be instance-specific.