Add CIDR support for expr (#62)

* feat: add cidr support for expr

* docs: add example for cidr

* minor code tweaks

---------

Co-authored-by: Toby <tobyxdd@gmail.com>

ruleset/builtins/cidr.go

@@ -0,0 +1,18 @@
+package builtins
+
+import (
+	"net"
+)
+
+func MatchCIDR(ip string, cidr *net.IPNet) bool {
+	ipAddr := net.ParseIP(ip)
+	if ipAddr == nil {
+		return false
+	}
+	return cidr.Contains(ipAddr)
+}
+
+func CompileCIDR(cidr string) (*net.IPNet, error) {
+	_, ipNet, err := net.ParseCIDR(cidr)
+	return ipNet, err
+}