Use Cobra functionality to validate arguments

[elhimov]

Closes #1125

[dmyger]

As for me, code with switch len(args) { was a bit clear to understand and support.

[oleg-jukovec]

Yes, it is. Switch instead of multiple if-else it is a Golang pattern. Even something like:

switch {
case foo < 1:
case bar > 2:
case baz == bar && baz == 3:
default:
}

[elhimov]

As for me this approach is simpler.
Consider them as 2 independent if-else. Each of them just deals with single corresponding parameter.
We don't need to validate program here anymore because its validation was moved to the dedicated validation function (as well as we don't need to duplicate that validation code with error handling in two cases). All we need here is to deal with 2 parameters and it could be done with 2 independent if-else.

[elhimov]

Just in case an important note: it wasn't just changing switch to if-else. The original switch-implementation did both: validated the existent arguments and obtained the missing ones. This PR follows the approach provided by Cobra Framework and put the validation part into the dedicated function. After that the only thing left to do is obtaining the missing arguments (if any). And it can be done completely independent for every of them with simple if-else, so no need in switch.
I'm clearly understand that switch in Go is a fancy thing that allows much more than switch in C, but here just not the case.

[dmyger]

Please update comment for the field, too. It should start with the field name.

[elhimov]

Done.