Added a command line option to print singleton entities.

For some processing tasks, you really want to identify *all* entities, even 
those with only one mention. 

Invoke this option by the property
```
printable.printSingletonEntities = true
```

By default, this is set to <tt>false</tt>: the current behavior, of printing only
non-singleton entities, is respected.

All unit tests pass; this is public domain.