Syntax table entries for non-alphanumeric chars

See #410 

List of characters generated with the unicode-fonts package
```
(mapc
 (lambda (block)
   (when (and (string-match-p "operators\\|punctuation\\|symbol\\|arrow"
                              (car block))
              (not (string-match-p "alpha\\|letterlike" (car block))))
     (insert
      (format "\n%-45s ; %s"
              (prin1-to-string
               `(modify-syntax-entry (cons ,(nth 1 block) ,(nth 2 block)) "_" table))
              (car block)))))
 unicode-fonts-blocks)
```

I hope I didn't miss any important characters.