Added option for ignoring dead_code
Add struct for controlling behavior of lexer
Change handle_error signature

Since `self.handle_error` always takes an ErrorToken, we could just
create the error in `handle_error` function.
Add configuration to XmlLexer
Add err_buf handling to XmlLexer

`err_buf` will be used to replace `err_token` that where previously
generated for error handling
Remove infinite recursion from Doctype
Ignored parser example executable