feat(tagsInput): add ability to disable the input

Opted not to use "ng-disabled" because this would have side-effects on the directive element itself (adding the disabled attribute) which isn't really needed or intended. Instead, there's a new 'disable-input' property. I'm not sure if this is the best naming but I'll let others decide.

Styles have been added for both the bootstrap and non-bootstrap version to represent the disabled state and a CSS solution is used to hide the input and the remove buttons by default. The remove buttons could be re-shown and would still work - the developer would have to use something like pointer-events: none; if they wanted to show but disable the remove buttons (this is what bootstrap does).

The input is also disabled using 'ng-disabled', in case a developer wants to keep it shown again (since we can't prevent users from using it with CSS as far as I know).

Closes #102