Fix headline tags, table pretty printing and multiline links

- we can't just look at the len of the string (~ #bytes) - that breaks down for
  tables containing characters consisting of multiple bytes. This handles
  more (still not all) cases and is good enough for now
- add _ to allowed tag chars - also require space between headline and tags
- links (link itself, not the description) spanning multiple lines are not
  supported - otherwise we would have to take care of splitting link and adding
  indentation for org pretty printing - and that sounds like such an edge case
  that it seems cleaner to forbid them

org/testdata/tables.pretty_org

@@ -9,6 +9,14 @@
 |---+---+---|
 | 1 | 2 | 3 |
 
+#+CAPTION: table with unicode characters
+| Character | Org       | Rendered HTML |
+|-----------+-----------+---------------|
+| Hyphen    | =a - b=   | a - b         |
+| Ndash     | =a -- b=  | a – b         |
+| Mdash     | =a --- b= | a — b         |
+| Ellipsis  | =a ... b= | a … b         |
+
 #+CAPTION: table without header (but separator before)
 |---+---+---|
 | 1 | 2 | 3 |