Add a --quiet/-q command line option.

When running a lot of tests that cause a lot of output by Emacs, there is a lot of noise on the screen. This patch makes ert-runner capture output until there is an error, at which point it will emit all captured output. If there are no errors, it will simply emit a success message.

Sadly, `write-region` circumvents all Emacs output mechanisms, "Wrote /some/file" messages still show up.

Oh, and I'm happy to rework this patch if you dislike the style or the way it works. Just let me know.