Get URL for link labelled X

Feature request

Add a new method that allows the user to specify a link label, of which the method returns the URL that the label is linked to.

e.g. if I have an email with 2 links:

1. `First link`
2. `Second link`

It would be great if we could do something like: `$I->getUrlForLink('Second link')` to get the URL from the second link.

Feel free to make the method name anything you like that makes sense.