Allow user to configure binary file extension

[cobcobs]

Addresses #77

[searleser97]

Instead of adding this behavior as default, let's better add an option in the configuration file that lets the user decide if they want to use .exe extension when using Unix systems

[cobcobs]

I have rewritten it to allow the user to specify a file extension for the binary (it defaults to exe) via the following in cpbooster-config.json:

{
  "executableFileExtension": "",
}

It also accounts for the user including unnecessary periods (e.g. "executableFileExtension": ".exe"), but I do not know if that behavior should be kept as it isn't consistent with the behavior of preferredLang.