Configuration

Configuration is documented on a per-use case: if a class uses a certain
value from the configuration object, it has to define its name and what the
value is used for.  This means decentralized configuration declarations that
are nevertheless error checking and a centralized configuration object.

Directives are divided into namespaces, indicating the major portion of
functionality they cover (although there may be overlaps.  Please consult
the documentation in ConfigDef for more information on these namespaces.

Since configuration is dependent on context, most of the internal classes
require a configuration object to be passed as a parameter.  However, a few
make this optional: they will supply a default configuration object if none
are passed.  These classes are: HTMLPurifier::*, Generator::generateFromTokens
and Lexer::tokenizeHTML.  However, whenever a valid configuration object
is defined, that object should be used.