The caller can take care of the armor. For consistency move the
responsibility to close the armor to the caller, and make the stream
Writer not propagate Close.
This also will also allow us to spin the armor implementation out into
its won package that imports format, without getting an import loop from
format.Parse magically invoking armor decoding.
Less magic in the API, more magic in the CLI.
No need to tie ourselves to GitHub.
The redirect is not set up yet, but as long as there is a replace in the
go.mod the tool can't be installed with "go get" anyway.
Not using age-tool.com because A) I don't actually like the domain and
B) it should be about the spec not the specific implementation.
