feat(dropdown): add 'target' attribute to links generated in dropdowns

There are cases when one needs to explicitly specify anchor `target` attribute, for example to bypass angular router and make actual server request like described [here](http://stackoverflow.com/questions/11580004/angular-js-link-behaviour-disable-deep-linking-for-specific-urls)
This PR adds the possibility to specify `target` property in dropdown datasource.

P.S. I've added test but unfortunately can not run it due to huge amount of errors on `npm install`.