Run tag name through escape filter to avoid invalid HTML

If a tag contains characters like <> or &, we currently generate invalid HTML.
This is easily fixed by sending the tag through the jinja escape filter.

(This bug is not theoretical, I hit it when using C++ template names for tags,
 like "boost::variant<>".)
add lang support for drafts (#826 & #1107)

Fix #826
Fix #1107
Merge pull request #1201 from hrbonz/draft_lang

Add language support for drafts
Merge pull request #1267 from mlang/escape-for-html

Run tag names through escape filter to avoid invalid HTML
Minor improvements to Settings docs
Add the setting SLUGIFY_ATTRIBUTE