add 'width' parameter to 'figure' shortcode
Document manual breakpoint of .Summary

The use of <!--more--> to set the breakpoint for the generated page summary is mentioned in a release note, but not in the doc itself.

Very useful - and it leaves the formatting in place.
Minor proofreading corrections to Hugo docs

- Add backticks and commas where necessary
- Remove some trailing whitespace
- Add front matter example in TOML
- Fix typo in one of the tags in Showcase
- Add 多说 (Duoshuo) as an alternative to Disqus
- Use internal links (i.e. without gohugo.io) where possible
- Use a colon to set off an example
- Change "it's" to "its" where appropriate
- Use typographical (i.e. curly) apostrophe on the front page
  where appropriate
- Capitalize "Github" as "GitHub"
added docs for 'intersect' and 'in' template functions
Add nil-check to Intersect

The Intersect template-method would fail if one or both of the lists were nil (post vs page; post has tags, page has not).

This commit adds a nil-check and returns an empty result if any of the inputs are nil.

See #537