Add validators for token input / adding tokens

[carsten-wilhelm]

New functionality:

• token validators - once when input is sent (on Enter) and once the token is added to the list
• option to only allow unique tokens
• option to search suggestion within suggestions (not only at the beginning)
• error messages for validation

[carsten-wilhelm]

Thanks for your token autocomplete project - we used it in one of our internal projects and added some functionality. Maybe it's useful, so I created a pull request.

[sabieber]

Thanks for your pull request and the proposed feature. I will have a look at it in the next couple of days.