Feature/Property discovery

• There's no way to find out what features and properties a given XMLReader recognizes.

• Solution: Define two new read-only properties:

  http://xml.org/sax/properties/supported-features

  The value of this property is an array of Strings containing the names of all the features supported by this XMLReader.

  http://xml.org/sax/properties/supported-properties

  The value of this property is an array of Strings containing the names of all the properties supported by this XMLReader.

• Or perhaps a method instead of a property?