allow restriction of destination ip address

[exaV]

See #49

We no longer throw an error if we connect to IP address while a
ALLOWED_DEST_FQDN is set, instead we check that the destination IP address matches the pattern. This is the simplest possible change that allows users to restrict which IP addresses can be connected to. A more elaborate change would be to introduce a new environment variable that reflects that both IP addresses and domain names can be restricted. I don't mind either solution.

[bobpaul]

When I checkout the commit before this, it builds fine. When I checkout this commit, I get

$ CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -ldflags '-s' -o ./socks5
# github.com/serjs/socks5-server
./ruleset.go:23:29: invalid operation: req.DestAddr.FQDN != nil (mismatched types string and untyped nil)
./ruleset.go:24:62: invalid operation: cannot indirect req.DestAddr.FQDN (variable of type string)
./ruleset.go:26:62: invalid operation: cannot indirect req.DestAddr.IP (variable of type net.I

[exaV]

@bobpaul I fixed the compilation error. Thanks for pointing it out.