add option to prevent tags duplications

[Idan Gozlan]

right now, if user enter the same tags twice or more, it's not preventing from him do that, we should have an option to prevent duplications :)

[Aakansha Doshi]

@idangozlan there is already a prop onlyUnique which if set to true will prevent duplicates.
By default this prop is false hence duplicates are allowed.
https://github.com/olahol/react-tagsinput#onlyunique

[Idan Gozlan]

Thanks :)