Wrong case for json options

[codebien]

We are using camelCase for the json options in k6 core outputs, ideally, we should use the same in this project to be consistent.

The suggestion is to check the Config fields and fix the current options with the wrong case.

[mstoykov]

I also think that the "argline" ones should be camelCase as well - they are somewhat mostly so in k6 internally already although there are some ... exceptions ... that proves the rule :P