#20 partial encTeX support

[Luis]

Do you mind adding encTeX's multibyte primitives (\mubyte, \endmubyte, \mubytein, \mubyteout, \mubytelog, \specialout, \noconvert) into the engine? Though they are not strictly needed to support UTF8, they come in handy to make other replacements; I've came to depend on them to implement a parser for lightweight markup on my files. \xordcode, \xchrcode and \xprncode are not required.