option to supply list view position function

I had a use case where I needed to position the list myself instead of following the cursor. My textarea was pretty small (in a narrow column) and near the bottom edge of the window and I wanted the list to display in a specific position.

I'm hoping you'll see that this is a simple but useful change to consider.

Usage example:

```js
$('textarea').textcomplete([
  { // emoji strategy
    match: /(^|\s):(\w*)$/,
    search: function (term, callback) {
      var regexp = new RegExp('^' + term);
      callback($.grep(emojies, function (emoji) {
        return regexp.test(emoji);
      }));
    },
    replace: function (value) {
      return '$1:' + value + ': ';
    }
  }
], {
  listPosition: function (position) {
    this.$el.css({
      top: 'auto',
      bottom: 50,
      left: 0
    });
    return this;
  }
});
```

And if you think there's a better way to implement this, I'm happy to update this PR. :+1: