Allow password to be over stdin without a prompt

I'd recommend a new minor version bump after this


```
dave @ [ bahamas10 :: (Darwin) ] ~/dev/passhash (git:dave-allow-stdin) $ echo -n password | ./passhash.js -t sha-512 -u test -i 50 -s 
test:ULa8EtE0bAZ0hgFbKwzW+dFG5DUkic1MBRQUU/qqvZE30NdPu4HIyZeNBHPMmvjD2q1EUGNTa5X/qQVILVqaT1rTxgijsWXTaPiCbQQkBAEwS5S1e3GIH8/p3Kokfdwr6iIhIs2oTSoGnb+L+bHbBhd7UgJSeC2yhpbSe3YA4Ag=:c7a96cedca54680ce9d579fc71ba04a3dd6ec55136ed64103b24f8213bbe0389570d59722adb81dd8ed6dc2175f7eddfa431ab14e66137f3882c13ff4817a049:50
```