Add -o FILE / --output FILE option to redirect output

As discussed back when we moved `--output-type` to `-O` to make room for it, add a `-o` option to specify an output file without needing `>FILE` shell redirection -- thus avoiding needing to quote shell metacharacters when writing scripts, etc.

As a pull request, so that you can regenerate _bcftools.1_ when merging it should you wish.