Added flexibility for autocomplete rules

Hey mizzao,

Thank you so much for this library, I have used it incessantly in all of my Meteor projects since finding out about it. I have used a forked version for a while though, because I needed some added flexibility for features like Markdown link autocompletion.

I wanted to send over this pull request so that people can get even more use out of this library.

For example, I use the following configuration in order to trigger autocomplete on `](` and then automatically close the link with `)` upon a successful completion.

```
Template.prose_edit.settings = function() {
  return {
    position: "bottom",
    limit: 5,
    rules: [{
      token: '\\]\\(',
      replacement: '](',
      end_token: ')',
      collection: Proses,
      field: "url",
      template: Template.prose_url_title
    }]
  }
}
```