Allow password to be provided by stdin

[jeremyevans]

OpenBSD signify supports password to be provided by stdin:

$ echo -n password > pass
$ signify -G -p t.pub -s t.sec < pass
$ signify -S -s t.sec -m t.pub < pass
$ signify -V -p t.pub -m t.pub
Signature Verified

It would be great if signify-rs could support this as well, as it's makes automated use much easier (consider use of signify-rs in automated tests). Implementation wise, if stdin is not a terminal, read stdin to get the password. If stdin is a terminal, continue prompting for a password. Here's how OpenBSD signify implements password reading (readpassphrase(3) reads from stdin if RPP_STDIN flag is used): https://github.com/openbsd/src/blob/600ae3309934717b40f3a8f30104f26d08b69791/usr.bin/signify/signify.c#L266-L280