Refactor space handling of writers

I went through the issues of goorgeous and picked a few that seemed easy enough
to add (and added some fore as todos for later). That helped a lot and showed
some bugs / edge cases that required changes.

- the org writer wrote a lot of eol spaces and just removed it whenever
  String() was actually called. That worked until now but did not bode with
  rendering an empty headline - by removing ALL eol space we would render "* "
  back as just "*" -> not a headline anymore.
- the html writer had some special handling for line spacing inside paragraphs
  and list items - with the introduction of more blocks we need that handling
  everywhere.
  As browsers / html renderers are nice enough to collapse whitespace (and
  especially collapse "\s*\n" into " ") we can just write out the newlines and
  let the renderer take care of the rest.

org/block.go

@@ -32,7 +32,12 @@ func (d *Document) parseBlock(i int, parentStop stopFn) (int, Node) {
 		if parentStop(d, i) {
 			return 0, nil
 		}
-		nodes = append(nodes, Line{[]Node{Text{trim(d.tokens[i].matches[0])}}})
+		text := trim(d.tokens[i].matches[0])
+		if name == "SRC" || name == "EXAMPLE" {
+			nodes = append(nodes, Line{[]Node{Text{text}}})
+		} else {
+			nodes = append(nodes, Line{d.parseInline(text)})
+		}
 	}
 	return i + 1 - start, Block{name, parameters, nodes}
 }