Add interactive configuration

This adds a new --config flag and rearranges some config fields.

config/validation.go

@@ -0,0 +1,41 @@
+package config
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+)
+
+var (
+	domainReg = regexp.MustCompile("^https?://")
+)
+
+const (
+	minPort = 80
+	maxPort = 1<<16 - 1
+)
+
+func validateDomain(i string) error {
+	if !domainReg.MatchString(i) {
+		return fmt.Errorf("Domain must start with http:// or https://")
+	}
+	return nil
+}
+
+func validatePort(i string) error {
+	p, err := strconv.Atoi(i)
+	if err != nil {
+		return err
+	}
+	if p < minPort || p > maxPort {
+		return fmt.Errorf("Port must be a number %d - %d", minPort, maxPort)
+	}
+	return nil
+}
+
+func validateNonEmpty(i string) error {
+	if i == "" {
+		return fmt.Errorf("Must not be empty")
+	}
+	return nil
+}