internal/spf: Add an SPF package

This patch adds a package for evaluating SPF, as defined by RFC 7208
(https://tools.ietf.org/html/rfc7208).

It doesn't implement 100% of the RFC, but it coves enough to handle the
most common cases, and will fail open on the others.

cmd/spf-check/spf-check.go

@@ -0,0 +1,20 @@
+// Command line tool for playing with the SPF library.
+//
+// Not for use in production, just development and experimentation.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"net"
+
+	"blitiri.com.ar/go/chasquid/internal/spf"
+)
+
+func main() {
+	flag.Parse()
+
+	r, err := spf.CheckHost(net.ParseIP(flag.Arg(0)), flag.Arg(1))
+	fmt.Println(r)
+	fmt.Println(err)
+}