14 bug fixes and added OASIS ODF Validator to CI

v0.5.1

Fixed 7 bugs in template engine (tag boundary parsing, nested empty spans, meta.xml processing)
Fixed 7 bugs in reader (single-quoted attributes, numeric character references, CharStyle tri-state, styles.xml integration, HTML5 charset)
Added OASIS ODF Validator to CI — generated output now validated against the ODF spec on every push
410 tests passing