urlRel2abs() prepended an extra "." when it saw "./X", turning the
relative path into "../X" and silently moving up a directory. As a
result, raw HTML <img src="./imgs/run.png"> inside a README rendered
under /r/<repo>/<file> resolved to /r/<repo>/imgs/... instead of
/r/<repo>/<dir>/imgs/..., so the image 404'd. Markdown image syntax
went through marked-base-url and was unaffected.
Strip the leading "./" instead so the relative path concatenates
cleanly with baseUrl.
Fixes#346.
marked v12 dropped the headerIds option, so headings rendered with no
id attributes and links like [Releases](#releases-and-contributing)
silently failed to scroll. Add a heading renderer that emits a
GitHub-style slug id, with a numeric suffix for duplicates within a
document.
Fixes#390.
tdurieux