feat(tagsInput): Add addOnTab option

The option addOnTab defaults to false, which should maintain current
behavior. When addOnTab is true and you hit tab with no text entered,
addOnTab option advances to the next field (like normal html form).
When you hit tab with text entered, addOnTab option completes the tab
AND leaves you inside the current tagsInput

Basically make ngTagsInput like gmail emailTo address tags.