Piping input to ledger program instead of providing a filename

[errge]

This ticket is a feature request, not a bug report.

When using ledger-mode together with an encrypted (GPG) file, the mode is trying to run ledger -f foobar.gpg, which will obviously not work, since the file is encrypted. An alternative would be to provide an option (or maybe even make it default?) that would cause ledger-mode to communicate using stdin instead of a file.

So when any operation that requires a ledger-file (e.g. account listing, reports, check, etc.) is ran, it's standard input would be provided with all the text between (point-min) and (point-max). On the ledger side, the command line has to be modified to be ledger -f - instead of ledger -f filename.

This would solve other similar use-cases (not just encryption): e.g. editing a remote file over TRAMP, or a file inside a ZIP, it would be cool to use the locally installed ledger binary with a pipe.

Sorry for not providing a pull request right now, but I still wanted to log this idea, in the hopes that somebody else will work on it.