A pattern attribute, edit a tag and replaced class by classes.

Hi! I did 3 commits to do some small changes that I think that could be useful. The changes are:

* A new `pattern` attribute. With this attribute is possible to indicate a regex that a tag should match _before_ is added to the array of tags. I'm using it to match email addresses, like this:  
`pattern="^[a-z]+[\w\d_\.-]+@[\w\d_\.-]+\.[a-z][a-z]+$"` <br><br>This way, if I type `john@yahoocom` (note the missing dot) and then press comma, the tag is **not** added. (by default, the required regex is `.*` so if someone does not use the pattern attribute, this person will have no problems at all)

* Right now, if I press backspace on the empty input, the previous tag is just deleted. But what if I made a mistake, like only one letter is wrong? I would have to recapture the entire tag. With this small change, the deleted tag goes inside the input tag. 

* If I use a class, the class is applied to the `<tags-input>` AND the inner `<div>`. Trying to use the `form-control` of bootstrap make it look broken. I think is better if the classes are specified using a different attribute than `class`, therefore I did a small change so it uses `classes` instead.


All of this are small changes. From the 3 changes, I think the one about `pattern` is the most useful.

Is up to you use all 3, only one of them or none of them.

Btw, thanks a lot for this input control! Is going to be really useful to me! :)

Regards.