Teedoc

* Add 404.html template, support i18n

* Add i18n support for plugins and templates
* Add comment plugin teedoc-plugin-comments-gitalk
* Add print page support
* Add anchor for titles
* Add warning log for wrong sidebar item
* Fix bug: not auto refresh page when content changed in previwe mode
* Fix search index file too large bug
* Fix navbar list item z-index error
* Fix toc smooth scroll bug when id is escaped charactors

* Add layout template customize support(Jinja2)

* Change markdown parser from markdown2 to mistune, now build faster at least 2x
* Ignore .git folder in file watcher

* Optimize file watcher, fix rename in file browser can't trigger event issue
* Fix error when copy error(not found error)

plugin theme_default: Add image viewer for img item

* Add summary2json and summary2yaml command for gitbook sidebar file
* Add sidebar splitter, you can set `sidebar_width`(default sidebar width) config for plugin `teedoc-plugin-theme-default`, e.g.:

"teedoc-plugin-theme-default": {
"from": "../../plugins/teedoc-plugin-theme-default",
"config": {
"env": {
"sidebar_width": "300px"
}
}
}