Closes #11.
AttributedString(markdown:) sets .link to the raw target, so
[ARCHITECTURE.md](ARCHITECTURE.md) became a scheme-less URL that openURL
cannot open — the text rendered blue and the tap did nothing.
MarkdownView now takes the same OrgRenderOptions ReadmeView already builds
for the org path and rewrites relative targets through OrgSwift's resolver
(krz/org-swift!8), so a relative link in a Markdown README reaches the same file
page an org one does. Bodies with no file to be relative to — issues, merge
requests, releases — pass no options and are unchanged.
Verified in the simulator on krz/org-swift: "See ARCHITECTURE.md." now opens
gitbay.org/krz/org-swift/blob/HEAD/ARCHITECTURE.md. Full suite green, including
three new cases over path, ./path, ../path, /abs/path, and the
no-options passthrough.