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

Section assets

This reverts commit c7156a84f0.

Fix path handling on windows

Otherwise Path won't know what to do.

Delay turning paths into string. Instead, use methods on Path/PathBuf
to manipulate paths. This way the Rust Path implementation gets to deal with
all werid cross-platform issues.

This should fix #359

Next version