Avoid confusing error message with duplicated formats.

If one specifies the same formatter and out stream (eg. '--format html
--out cucumber/report.html') in both the 'std_opts' part of
'config/cucumber.yml' and the command line, the error message was the
following:

"All but one formatter must use --out, only one can print to each
stream (or STDOUT)".

This was really bewildering in most cases, since the user usually wants
to use the command line to try/verify/rerun things and the message is
not helping him/her understand the cause of the problem (remove the
duplicate format with the same output, duh!).

To avoid the confusing error message, the (format, output) pairs are now
uniqued, which I think is consistent with the user's wishes.