Configurable extension

[tomeus]

When choosing format as json it could be a fine generating file with json extension.

Maybe rename jsPath and jsFile to outputPath and outputFile, add outputExtension.

[martinlindhe]

I would welcome a patch if you find it needed, but just add outputExtension.

We want to avoid breaking existing configurations so I rather keep the jsPath and jsFile names for now, even they are a bit annoying in hindsight.

[tomeus]

Hi. I have found the time and I return to this. I review the implementation again and say that this configuration parameter will only be taken for generateMultiple. Does it make sense to you? I'm also new in contributing, do you have any special pull request requests?