RFC1918 IPs shouldn't be discarded by default

[hrefhref]

Hi,
I started using this plug in applications that are only deployed in a VPN/LAN. However, it discards all the clients IPs as they are RFC1918 IPs (mostly 10/8, some 192.168/16).

It would be nice to allow an option to not discard some of theses reserved networks. Maybe even sub-ranges if that makes more sense (e.g. our VPN is 10.42/16).

[ajvondrak]

See, I was paranoid about this exact issue while hard-coding the private networks in

remote_ip/lib/remote_ip.ex

Lines 55 to 64 in 827762a

	# https://en.wikipedia.org/wiki/Loopback
	# https://en.wikipedia.org/wiki/Private_network
	@reserved ~w[
	127.0.0.0/8
	::1/128
	fc00::/7
	10.0.0.0/8
	172.16.0.0/12
	192.168.0.0/16
	]

I think I was just copying what Rails was doing: https://github.com/rails/rails/blob/94b5cd3a20edadd6f6b8cf0bdf1a4d4919df86cb/actionpack/lib/action_dispatch/middleware/remote_ip.rb#L31-L42

What do you think about maybe lifting @reserved out into another keyword option, where the default is the currently-hard-coded list? Then you could init the plug with reserved: [...]. I guess for your use case a whitelist might make more sense, though. 🤔