Support custom date format in gantt-tooltip directive.

#### Motivation

I am using the angular-gantt module to display availability of physical things. It's like a reservation system. (The Angular-Gantt module is super awesome. Nice job!)

My things are reserved by the day, not by the hour or minute. When I hover on one of my item's reservations (a row's "task"), I see the date and time. I want just the date.

#### Solution

This date is defined in a "ganttTooltip" directive. Nice design! Even better, this date is passed through a date formatting filter. All we have to do is change this to a variable and set the variable from somewhere. Where do we pass in the custom date format?

I see two places to pass a custom date format:

1. Create a new ganttTooltip directive attribute, called  "dateFormat"
2. A property on the object specified by the `ngModel` attribute
3. (Other?)

Which is better? If I get a quick response, I can update my pull request to use a different solution.

This Pull Request implements option 1. I customized my gantt template to include a `date-format` attribute on the `gantt-tooltip` element.