feat(tooltip): add enabled option

Add a new option to the tooltip that enables or disables the tooltip to show. Related to #609, #871 and #1174 I think.

Hope this serves as a basis for all other tooltip derived directives, so putting it here to get some feedback before propagating to other directives:

- I don't know if this should be concerned with multiple directives per element, sort of like bsShow does? 
- The attribute value checking might need some improvement?
- Should we use attribute.observe or scope.watch?