Add --number option to run a single test by its id

Do not check input validity, e.g. input `>` number of tests, because
other inputs don't check it either, e.g. `--pattern`.

If given, the summary line will be like:

    1 passed, 0 failed, 0 errored, 509 skipped
Merge pull request #247 from cirosantilli/run-single-test-by-number

Add --number option to run a single test by its id