Add "multiSelect" field type

[rowanmoul]

I implemented this myself with very minimal effort. Would be great to see this in here by default.
This is my implementation:

formlyConfig.setType({
                name: 'multiSelect',
                extends: 'select',
                template: '<select multiple class="form-control" ng-model="model[options.key]"></select>'
            });

I would gladly turn this into a pull request, but I'm fairly new to Javascript and I'm not sure how I would write tests for this.

[rowanmoul]

Here us a JS Bin with an example:
http://jsbin.com/reheko/edit?html,js,output

[kentcdodds]

Awesome! I'd happily accept a PR for this. I'm not sure how I'd test this one either. But I'll go ahead and accept a PR for this even without tests. (This repo isn't set up with tests yet anyway).