onlyAllowAutoCompleteTags option

Adding the 'onlyAllowAutoCompleteTags' option to tag-it.
- Pressing tab with content that doesn't match a tag will remove that
content and tab out of the input.
- Pressing tab with content that matches a tag case insensivity and the
caseSensitive option is set to false the tag is updated to be the source
value.

Misc fixes:
- Autocomplete menu closes on backspace press.