new modifier: ListItemIs

```
useful new modifier for multi-value fields

E.g.:
book1 tags: tag1, tag2, otherTag
book2 tags: tag1, tag2, anotherTag

<<Tags.Contains:other>>
returns book1 and book2

<<Tags.ListContains:anotherTag>>
returns only book2


```

Original issue reported on code.google.com by `rainer.c...@googlemail.com` on 31 Mar 2013 at 4:15
