Added multiSelect input type as extension of select type.

[rowanmoul]

PR for #47
I added it to select.js rather than make a new file. Let me know if you would like it to have it's own file.

[kentcdodds]

Thanks for doing this. The problem with this solution is you don't get the same functionality with the multi-select type that you do with the normal select (like custom ngOptions). So I think a better solution would actually be too combine the two together. You can add a property to the ngModelAttrs to have the multi select be added when a property is set to true on the options... I may look into it further unless you want to give it a shot.