It would be great to have the helper name passed to `blockHelperMissing`

This may not be the intended use of `blockHelperMissing`, but I'm finding myself wanting to treat it like a regular "method missing" function to dynamically generate helper content.

This could easily be achieved by passing `this.lastHelper` as the final param to the `blockHelperMissing` callback. I believe here is where it would need to be added:
https://github.com/wycats/handlebars.js/blob/1.0.0/lib/handlebars/compiler/compiler.js#L649

This would allow me to do stuff like I show in the examples below (dynamically look up the data to iterate over by parsing the helper name):

```
<ul>
  {{/eachArticle}}
    <li>{{title}}</li>
  {{#eachArticle}}
</ul>
```

Or even more elaborate:

```
<ul>
  {{/eachArticleByDate}}
    <li>{{title}}</li>
  {{#eachArticleByDate}}
</ul>
```

If this is functionality you'd be willing to have in Handlebars, I can submit a pull request with the changes.