Combine cobra/viper configurations with any zcash.conf settings

[benzcash]

Currently the configurations are separate, it would make more sense to combine them!

For example:

If the user specified 3 of these but forgot one, then we'd get the values from the conf file, but would that be confusing to the user? Maybe throw an error if any of these 4 are specified on the command line but not all of them.

OR, a really nice thing would be to take the values from the file (if it's specified, I understand that the goal of this PR is that the conf file doesn't have to be specified, doesn't have to exist on this machine), and then the command-line options override the values read from the file. That's how zcashd works -- command-line arguments override config file values.

But if you think that's too much, that could be a separate PR.

One really minor suggestion is to refactor this test (if all the strings are non-empty) into a separate function that can be called from two places (here and at line 69).

Originally posted by @LarryRuane in #284