Allow configuration of path to keychain file

[Tenzer]

This adds the -keychain parameter to all commands which can be supplied a path to where the user want 2fa to place the keychain and read it from.

The default value of ~/.2fa can also be overruled by setting a path in the environment variable named KEYCHAIN, so it doesn't have to be supplied on every invocation.

Priority order is:

1. The -keychain argument.
2. The KEYCHAIN environment variable.
3. Default value of ~/.2fa.