Added support for functions as templates

[NatalieWolfe]

This adds support for passing in a function as a template so you can use other a simple function of your own with no template engine, or a precompiled template (such as you get when using Jade).

[jharding]

This is intriguing. Currently if you wanted to use something like Underscore for templating, you'd have to do something like:

$('.typeahead').typeahead({
  // ...
  template: '<%- value %>',
  engine = {
    compile: function(template) {
      var compiled = _.template(template);

      return {
        render: function(context) { return compiled(context); }
    }
  }
});

This is somewhat tedious and the solution in this pull request might make this a little easier to accomplish. I just need to think about this for a bit and make sure there's no drawbacks, but my gut reaction is that it should be fine. Assuming I don't think of any drawbacks, this will go out in v0.9.1. Thanks for the pull request!

[jharding]

Updated your changes to work with v0.9.0, let's move any future discussion of this topic over to #137.