UX: Autogenerate output filename

[simonlbn]

For standard interactive use, it would be really nice if age autogenerated the output filename.

To be specific, age -r age1.... myfile age would write output to myfile.age, or something like that.

This would be similar to what gzip (and more) does for compression where gzip myfile compresses the file to myfile.gz.

Currently since I have to manually type output file, accidentally overwriting the input file is when running a command like age -r age1... myfile > myfile is a real possibility. Supporting this would also mean less typing and copy/pasting when using age so in my opinion be a UX improvement.

Since the current default is just output to stdout, one way to do this without breaking existing uses may be a '-O' flag slightly similar to curl to auto generate filename, assuming you don't want to break current usage.

PS. I looked through previous Github issues and couldn't find a discussion about this topic.