R2L: negative rule post-processing, version 1

Given a `negativemarker`
```
EvaluationLink
   PredicateNode "negativemarker"
   ListLink
      word_instance
```
generate
```
NotLink
   AndLink
      ** links involving word_instance **
```

For example, in "I do not eat injera.", `word_instance` will be `eat@1111`, while "The cat is not small.", `word_instance` will be `small@1111`

Refer to
opencog/relex#87
opencog/relex#117