Add the ability to check for expected email attachments in MailContext

[chrisolof]

I would like to be able to test for the presence of an expected email attachment, in addition to correct to and subject fields. For example:

  ...
  Then an email has been sent to "[email protected]" with the subject "Foo" and the attachment "bar.pdf"
  ...

Or, in the case of multiple attachments:

  ...
  Then an email has been sent to "[email protected]" with the subject "Foo" and the attachments "bar1.pdf,bar2.pdf"
  ...

I intend to follow this up with a pull request.