Added amdRequire option to specify AMD dependencies

I've added a new option `amdRequire` to help when your Handlebars helpers are defined in a separate module that needs to be loaded prior to compiling your templates.

This option will help with this issue:
https://github.com/gruntjs/grunt-contrib-handlebars/issues/58

Simply put, instead of always generating this AMD header:
```js
define(['handlebars'], function(Handlebars) {
```

When you specify `amdRequire`:
```js
options: {
   ...
   amdRequire: ['handlebars.helpers']
}
```

Then the output will be this:
```js
define(['handlebars','handlebars.helpers'], function(Handlebars) {
```