fix(highlight): ng-bind-html-unsafe is not available in 1.2+

#### Issue
`ng-bind-html-unsafe` is not available in 1.2+ so we have to use `ng-bind-html`, but these need trusted data

#### Changes
1. Changed demo html to use `ng-bind-html-unsafe` to `ng-bind-html`
2. Filter now returns trusted html using `$sce` provided by angular
3. Updated test cases to convert the filtered value to html using `sce.getTrusted(sce.HTML, trustedHtml)` , before comparing with test phrase