attachTo is not working when passing a string

[menor]

First of all, thanks for the awesome work on the library.

And now the issue:

The documentation specifies that we need to pass a string to the attachTo key

But this wasn't working for me, I had to pass an object like this
attachTo: {element: '.add', on: true}
To make it work, I think this is related to the parsePosition method that returns null if the str argument is not an object with an element and on keys.

function parsePosition(str) {
  if (isObjectLoose(str)) {
    if (str.hasOwnProperty("element") && str.hasOwnProperty("on")) {
      return str;
    }
    return null;
  }

Didn't made a pull request cause I don't know if is a matter of fixing the code or the documentation. But creating this issue for reference.

I'm passing the attachment positions as tether options, maybe that has also something to be with the issue.

[menor]

closed by #131
Thank you!