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/headline.go

@@ -17,7 +17,7 @@ type Headline struct {
 }
 
 var headlineRegexp = regexp.MustCompile(`^([*]+)\s+(.*)`)
-var tagRegexp = regexp.MustCompile(`(.*?)\s*(:[A-Za-z0-9@#%:]+:\s*$)`)
+var tagRegexp = regexp.MustCompile(`(.*?)\s+(:[A-Za-z0-9_@#%:]+:\s*$)`)
 
 func lexHeadline(line string) (token, bool) {
 	if m := headlineRegexp.FindStringSubmatch(line); m != nil {