Validate all command option values in config files

[kumar303]

This is a stepping stone towards #176

If the developer runs a command like:

web-ext build

but their config file has an invalid option for an unrelated command, like:

{
  sign: {
    apiSekret: 'this-config-key-has-a-typo', 
  },
}

then it should still raise an 'invalid config option' error even though the developer did not execute the sign command.

[shubheksha]

This should be taken care of in #842

[saintsebastian]

@kumar303 i think this issue is solved as @shubheksha has mentioned

[kumar303]

Ah, indeed it is. Thanks.