html: Support pretty relative links

Hugo defaults to serving files with pretty urls [1] - this means
`/posts/foo.org` is served at `/posts/foo/`. This works because servers
default to serving index.html when a directory is specified and hugo renders
the post to `/posts/foo/index.html` instead of `/posts/foo.html`. To make
relative links work we need to (1) remove the fake `foo/` subdirectory from
unrooted links and (2) replace any `.org` suffix with `/`.

[1] https://gohugo.io/content-management/urls/#pretty-urls

org/html_writer_test.go

@@ -42,3 +42,30 @@ func TestExtendedHTMLWriter(t *testing.T) {
 		t.Errorf("WriteText method of extending writer was not called: CallCount %d", extendedWriter.callCount)
 	}
 }
+
+var prettyRelativeLinkTests = map[string]string{
+	"[[/hello.org][hello]]": `<p><a href="/hello/">hello</a></p>`,
+	"[[hello.org][hello]]":  `<p><a href="../hello/">hello</a></p>`,
+	"[[file:/hello.org]]":   `<p><a href="/hello/">/hello/</a></p>`,
+	"[[file:hello.org]]":    `<p><a href="../hello/">../hello/</a></p>`,
+	"[[http://hello.org]]":  `<p><a href="http://hello.org">http://hello.org</a></p>`,
+	"[[/foo.png]]":          `<p><img src="/foo.png" alt="/foo.png" title="/foo.png" /></p>`,
+	"[[foo.png]]":           `<p><img src="../foo.png" alt="../foo.png" title="../foo.png" /></p>`,
+	"[[/foo.png][foo]]":     `<p><a href="/foo.png">foo</a></p>`,
+	"[[foo.png][foo]]":      `<p><a href="../foo.png">foo</a></p>`,
+}
+
+func TestPrettyRelativeLinks(t *testing.T) {
+	for org, expected := range prettyRelativeLinkTests {
+		t.Run(org, func(t *testing.T) {
+			writer := NewHTMLWriter()
+			writer.PrettyRelativeLinks = true
+			actual, err := New().Silent().Parse(strings.NewReader(org), "./prettyRelativeLinkTests.org").Write(writer)
+			if err != nil {
+				t.Errorf("%s\n got error: %s", org, err)
+			} else if actual := strings.TrimSpace(actual); actual != expected {
+				t.Errorf("%s:\n%s'", org, diff(actual, expected))
+			}
+		})
+	}
+}