add unknown flags support

[TiboStev]

Make able to use unknown flag and make it accessible for future use. For a wrapper for example

[CLAassistant]

All committers have signed the CLA.

[cornfeedhobo]

There have been a few PRs related to the idea of tracking unknown fags. Can you explain why you chose this approach over #199?

[TiboStev]

I didn't see #199, I fell that using flags and VisitUnknowns() may fit better the API and make it more elegant if one needs to iterate over unknown flags. But in the end I just need a way to retrieve unknown flags so one way or another I'm fine with it.

[cornfeedhobo]

@TiboStev No worries. Design decisions are tough, that's why I had to ask. I've gone ahead and implemented #199 on my fork, but I like this branch because it's got a similar API and the changes to parsing are much more clear for the reader.

[cornfeedhobo]

@TiboStev Made a version of this on my fork. Feel free to check it out 😃

[TiboStev]

thanks for the review @cornfeedhobo I'll have a look

[kennytm]

There is also #187 besides #199 🤷