Need a way to disable entity parsing

[hanshuebner]

Currently, luaexpat always parses standard XML entities into their corresponding characters. While this is convenient for most applications, it inhibits creating programs that preserve entity encoding when serializing a document. expat provides for the XML_SetParamEntityParsing function to control whether it parses entities, and it would be useful to expose control over this functionality in LuaExpat, maybe as a parser:setentityparsing function.