add -g, --github <user> flag

[casperdcl]

Similar to https://github.com/S2-/sshenc.sh#encrypt-a-file-using-the-public-key-of-a-github-user

would simplify:

# old
curl -sL https://github.com/someusername.keys | age -R - -o output.file input.file
# new
age -g someusername -o output.file input.file

[str4d]

This was present in an early draft of the age CLI design, and I had implemented it as an unstable feature in rage. However, we decided over the new year to drop the "encrypt to GitHub users" feature. It relies on support for encrypting to recipients at HTTPS URLs, and there are many decisions that need to be made when downloading a file (e.g. what roots to trust?) that go beyond the APIs we want to focus on here, and are better suited to dedicated URL-fetching tools like curl or wget.

As you note, encrypting to a GitHub user in a single command can still be done with pipes. It can also be achieved with process substitution:

cat input.file | age -R <(curl -sL https://github.com/someusername.keys) -o output.file

[casperdcl]

er, why not launch a curl/wget process from age (throwing an error if the binaries are not found)?