Remove two unmaintained vendor packages `i18n` and `paginater`. Changes:
* Rewrite `i18n` package with a more clear fallback mechanism. Fix an unstable `Tr` behavior, add more tests.
* Refactor the legacy `Paginater` to `Paginator`, test cases are kept unchanged.
Trivial enhancement (no breaking for end users):
* Use the first locale in LANGS setting option as the default, add a log to prevent from surprising users.
* Add control for the rendering of the frontmatter
* Add control to include a TOC
* Add control to set language - allows control of ToC header and CJK glyph choice.
Signed-off-by: Andrew Thornton art27@cantab.net
wxiaoguang