add-signature to read from stdin

[ChevronTango]

Whilst working on #525 (comment) I found that I had to write the signature out to file and then read it in again when I called add-signature. It would be useful if I could avoid the disk IO and pipe the signature into the add-signature call.

# current approach
tuf add-signature --signature targets.json.signature targets.json

# proposed solution
cat targets.json.signature | tuf add-signature targets.json