Add email validation option, set to false by default

[martincarlin87]

Added an option validateEmails as I was using this for a CC list and didn't want any tags to be added unless it was a valid email address.

[lucknerjb]

Thanks for the PR however, a few things:

1 - There are too many whitespace diffs for this to be merged in
2 - validateEmails is too specific and is more of a use-case than an actual needed feature

This kind of validation can be added in the onAddTag callback.

At best, adding a validations key to the options would be an option but even that might be overkill.