html: Implement fat table rows (use tbodies to represent separators)

html does not support table separator rows as Org mode does. Emacs org export
simulates rows as defined by separators by wrapping all the rows between 2
separators into a separate tbody. The html spec is fine with that [0] so we
follow.

[0] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/tbody

org/html_writer.go

@@ -459,23 +459,31 @@ func (w *HTMLWriter) WriteNodeWithName(n NodeWithName) {
 
 func (w *HTMLWriter) WriteTable(t Table) {
 	w.WriteString("<table>\n")
-	beforeFirstContentRow := true
+	inHead := len(t.SeparatorIndices) > 0 &&
+		t.SeparatorIndices[0] != len(t.Rows)-1 &&
+		(t.SeparatorIndices[0] != 0 || len(t.SeparatorIndices) > 1 && t.SeparatorIndices[len(t.SeparatorIndices)-1] != len(t.Rows)-1)
+	if inHead {
+		w.WriteString("<thead>\n")
+	} else {
+		w.WriteString("<tbody>\n")
+	}
 	for i, row := range t.Rows {
-		if row.IsSpecial || len(row.Columns) == 0 {
-			continue
-		}
-		if beforeFirstContentRow {
-			beforeFirstContentRow = false
-			if i+1 < len(t.Rows) && len(t.Rows[i+1].Columns) == 0 {
-				w.WriteString("<thead>\n")
-				w.writeTableColumns(row.Columns, "th")
+		if len(row.Columns) == 0 && i != 0 && i != len(t.Rows)-1 {
+			if inHead {
 				w.WriteString("</thead>\n<tbody>\n")
-				continue
+				inHead = false
 			} else {
-				w.WriteString("<tbody>\n")
+				w.WriteString("</tbody>\n<tbody>\n")
 			}
 		}
-		w.writeTableColumns(row.Columns, "td")
+		if row.IsSpecial {
+			continue
+		}
+		if inHead {
+			w.writeTableColumns(row.Columns, "th")
+		} else {
+			w.writeTableColumns(row.Columns, "td")
+		}
 	}
 	w.WriteString("</tbody>\n</table>\n")
 }