Updated handling of entity encoded characters. This affects certain multibyte chars like `\u202F`, etc:

* added `:html_entity` span_parser to Kramdown::Parser::KramdownRepositext so that html_entities are parsed in kramdown and can then be converted as needed.
* when converting to latex for PDF, we rely on kramdown's `convert_entity` method to convert the entity encoded characters to the decoded chars (unless we override this method where needed, e.g., in Kramdown::Converter::KramdownVgr where we want the entities encoded)
* removed manual decoding of entities in Kramdown::Converter::LatexRepositext. This isn't necessary any more now that entities are parsed by kramdown and converted by the `convert_entity` method.
Updated interface for Validation::Validator::Utf8Encoding spec to operate on IO rather than string