Replace fold/reduce over Result::and with collect

Not only is this tidier, but it avoids making these iterators
Rayon-specific.

Use a HashSet for detecting orphan pages

This offers a big performance bump for large sites:

  small-kb (100 pages)
    before: 530ms
     after: 500ms

  medium-kb (1000 pages)
    before: 2.9s
     after: 1.5s

  huge-kb (10,000 pages)
    before: 150s
     after:   9.1s

Fix #383

Closes #418

Closes #388

Closes #417

Closes #403

Change highlighting to not include duplicated background colors

Fix #412

Next version

Update pagination for test_site posts

Updates docs to match proper toml syntax.

link_checker: Set Accept header

Fix rendering benchmarks.

As mentioned in #381, crates.io 404's any request without an Accept:
text/html header.  It 200's any request with one, but at least
false-successes don't prevent checking any other links.

This also makes it easier to add a custom User-Agent if desired.

rustfmt and fix a clippy nit (unnecessary return) while I'm here.

Improve summary handling

As mentioned in #376

Push summary handling into Markdown parsing, identifying the presence
of one by giving its length in the rendered markup.

Hopefully a better fix for #376.

Also fix tests

Close #367, #372

Runtime syntax loading

Update page variable list for custom taxonomies

I love the new support for custom taxonomies! I got a little thrown off when updating my site, though, as the docs didn't specify how the page would expose the taxonomies to the template. Wasn't too hard to figure it out, but I figured I'd save the next person who tries it the effort :)

Section assets