Add Cancellable reporter

It will be used by the suite runner in order to support cancelling test runs.
Don't show aborted tests in the error detail list

If the user aborts overman tests, it's probably because he wants to see
failures that have already happened. Besides, we won't be able to report
anything sane here, since the reporter won't get any actual error.
Add support for showing aborted tests to the spec progress reporter
Add support for aborted tests to the Summary reporter