outfile added to reportbug.conf (from -o)

BTS report: #916985

This enables you to provide an outfile path in reportbug.conf, which is used as the default to options.outfile, which the -o option interacts with. This lets you set up reportbug to output to a file by default, which is helpful if your MUA isn't supported.

I have added this to the default reportbug.conf, commented of course, as well as the reportbug.conf manpage.

This change was inspired by https://blog.andresgomez.org/2017/02/28/how-to-use-debians-reportbug-with-evolution/#content