Add a password testing mode for use with password generators

[penma]

As detailed in the README:

Of course, if your EncFS volume was not created with predictable volume keys, but you assume that your password at that time was particularly weak (pet/family names, 12345, etc.) you can still try to guess the password by using standard password enumeration tools. A future version of this tool will include a mode that causes it to read passwords from standard input and check each of them. The same heuristics as for volume key enumration will be used, so false positives (correct checksum but incorrect key) can be eliminated. (Also, doing this in a single process will be slightly faster than spawning a new encfs process for each password to test.)