Add mpileup --output option to redirect output to a file
Add mpileup -o FILE as a shorthand for --output

Unfortunately -o INT already means --open-prob.  As a tremendous hack
for the sake of UI convenience and consistency (-o meaning --output is
very widespread), we choose between the two by whether the argument
looks like an integer.

Typical filenames (e.g. "foo.bam" or even "15.cram") don't, and
"-o ./123" or "--output 123" are documented workarounds if the user
actually wants a numeric filename.