Added ability to send in data from a Houston subscription to a custom template.

Not sure if this is how y'all would handle it.  Just my crack at it, and it seems to work for my application.  Addresses issue #177.  Haven't yet taken a look at manipulating data.

Usage:

```javascript
Houston.menu({
  'type': 'template',
  'use': 'signup_admin',
  'title': 'InviteUsers',
  'subscribedCollections': ['SignedUpUsers']
});
```

A bit of a weird method, `Houston._houstonize_custom_template_path` just stores the way the path is created.  I had to do this because in the `data` callback, the menu items don't yet have the `path` set when the browser is refreshed from the custom template URL itself.  So the path needs to be recalculated there, unless it is ensured to have happened beforehand.