Allowing literal commas in options

Hi! I'm the guy from this issue: https://github.com/itzg/go-flagsfiller/issues/31

If I had had noticed initially that you were the owner of *both* repos, I might have started here first. 🙂

`saml-auth-proxy` seems to do almost everything I need, except for allowing comma-bearing values to `-authorize-values`. Thanks so much for making this. And for being so responsive with the library!

I have zero experience with golang, but I'm willing to at least *try* to write the code to allow this. Would you be willing to accept such a change?

I could just hard-change `main.go:23` to stop splitting commas like I did for my own build as a stop-gap, but that doesn't seem the right thing. What would you suggest?

Thanks!

Start at main.go:23, where the issue reports that -authorize-values is split on commas, and inspect the surrounding option parsing. Check any existing tests for authorize-values before deciding how literal commas should be represented. Done means comma-bearing values passed to -authorize-values are accepted without being split unexpectedly.

Written by the indexing model from the issue text.