Ignore single word matches with threshold

Prevent false positives on single word matching when the user provides a threshold. 

This change doesn't seem to violate any expectations/contract as to how this gem works, but there could be users who are relying on the current behaviour for whom this would be a breaking change. For my use-case I'd rather have no match than a very distant match, but there may be others who prefer the current behaviour.
`threshold.nil?`