bgzip: allow specification of output name.

[ghuls]

It would be nice to be able to specify an output name when using bgzip>
Especially when the input comes from stdin (e.g output from some preprocessing of an input file).

$ echo test | bgzip /dev/stdin
[bgzip] can't create /dev/stdin.gz: Permission denied

# Now this is required.
$ echo test | bgzip -c /dev/stdin > test.gz

# This would be useful to have.
$ echo test | bgzip -o test.gz /dev/stdin