Added a Custom-Class option to Tooltips.

We had a need for custom classes per tootip (i.e. delete tootips to have a red background etc.) - so we've added a quick-fix with a CustomClass option.

i.e. `<div bs-tooltip title="Some great message" custom-class="awesome-custom class-here">Hover Me</div>`

Apologies if there is already an out-of-the-box way to do this with ng-strap (it seemed like a natural thing to want for customisation?). 

Otherwise: I hope its useful!

Kudos for creating the project,

D.