Support > :first-child and > :last-child

[ghost]

The following selectors seem to generate invalid XPath, crashing the whole parser:

With :first-child:

  .an-element > :first-child {
    /** Any rules **/
  }

With :last-child:

  .an-element > :last-child {
    /** Any rules **/
  }

The issue is closely related for these two. It causes an InvalidArgumentException to be thrown (at https://github.com/jjriv/emogrifier/blob/master/Classes/Emogrifier.php#L1532)

[joeworkman]

:not(.class) selectors also trigger this error.

[oliverklee]

@joeworkman I've entered #469 for adding support for :not(.class).