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/org.go

@@ -3,6 +3,7 @@ package org
 import (
 	"fmt"
 	"strings"
+	"unicode/utf8"
 )
 
 type stringBuilder = strings.Builder
@@ -268,7 +269,7 @@ func (w *OrgWriter) writeTable(t Table) {
 				if content == "" {
 					content = " "
 				}
-				n := column.Len - len(content)
+				n := column.Len - utf8.RuneCountInString(content)
 				if n < 0 {
 					n = 0
 				}