Allow configuring exceptions for var-shadows-builtin

[anderseknert]

If you're not using e.g. http.send or the new uri.parse functions and have no plans to do so, Regal should let you use names like http and uri as vars where you want to. I suggest we add a configuration option to var-shadows-builtin to allow a list of excepted names. While there's also a rule-shadows-builtin... that kind of shadowing feels more dubious to me, and I don't think there's reason to allow exceptions for that.

[anderseknert]

Something is off with the formatting there, but sounds good to me. I think we could have the list in config named something more specific than just “exceptions” though. Maybe except-namespaces or something like that? Should be some more rule that follow this already, but I’m on my phone right now so will have to check later.