Adding ability to load helpers through a glob pattern.

Adding loading of helpers with a glob pattern. This allows a user to specify the path to the helpers that need to be loaded, then they're loaded with the current plugin...

```js
assemble: {
  options: {
    helpers: ['./lib/helpers/**/*.js']
  }
}
```

Helpers can either be an object or a single `register` function. If `register` is on the object, then it calls the `register` function passing in the engine, otherwise, each method is registered as a helper....

```js
module.exports.foo = function(msg) { return msg; };
module.exports.bar = function(msg) { return msg; };
```
This will result in 2 helpers being registered

```js
module.exports.register = function(Handlebars, options) {
  Handlebars.registerHelper('foo', function(msg) {
    return msg;
  });
};
```
This will result in the `foo` helper getting register directly through Handlebars.

This PR is dependent on an update to `assemble-handlebars`, so it should be merged after that's been published to npm.