Add abilty to submit hash directly

[jzbz]

Add a way to pass dcrtime a SHA256 hash directly to be anchored as opposed to providing it with a file that it hashes and anchors. So instead of doing dcrtime -v -file <filename> something like dcrtime -v -hash <hash>.

[marcopeereboom]

I thought we had that magic but it turns out it was to treat a filename as a hash. So this is going to have to be slightly magical in order to keep that interface in tact. That said I agree we need this functionality.