Add fragment_cache helper. Add cache option for partial helper.
Add helpers: image_tag, favicon_tag, feed_tag
Update helpers: link_to, mail_to
Update tests
Add url_for helper
Add global variable: `hexo.env.args` for cli arguments
util.format.strip_html is renamed to util.format.stripHtml
New filter type: server_middleware
If root url is not `/`, redirect to the correct root url
Apply url_for helper to other helpers
Merge branch 'feature/relative-link' into dev

Conflicts:
	test/blog/package.json
Merge branch 'feature/fragment-cache' into dev
Merge branch 'feature/server-middleware' into dev
Remove unnecessary files