auth: Added cmdline option to specify passwords are bcrypted

[sum12]

bcryptpass cmdline informs the credentialStore that received passwords
should be hashed before comparing to the passwords obtained form the
authfile.

Closes #395

[otoolep]

Thanks for your PR. The functionality looks useful. However I think it's more complicated than it needs to be. There are two ways I think it could be simpler.

1. Instead of passing bcrypt-pass to rqlited when it starts up, just try the existing logic. If that doesn't work, then try the bcrypt hash check. Only if both fail is the access blocked.
2. Add a new field to the Passwords file, named, say, password_bcrypt. If this field exists check that, otherwise do the existing check. This change would be backwards compatible with existing files. Again you wouldn't need to pass bcrypt-pass to the binary.

This minimize the changes and means the way rqlited is launched is not changed.

[sum12]

@otoolep thanks for the input.
I have update the PR to the second option. Let me know what you think.

[otoolep]

Thanks @sum12, implementation generally looks good to me. -- I will merge this, and make a couple of changes on top of it.