Allow for setting always-auth with `--reg` CLI options for scoping.

@othiym23 not exactly sure where to put this, but I wanted to get your thoughts on the implementation details before finishing this up with tests etc. It seems like there is a larger API surface area for "nerfed" configuration in general, but that might be better suited to `npmconf`. 

I wasn't sure how far you wanted to go with this, but I wanted to put my money where my mouth is for this in #6230 and #6263