Add mpq extract util to repo (#746)

The gist with this script disappeared, adding to the repo since it's so useful.
Added some error handling/logging to the script.

utils/extract-mpq/doc.go

@@ -0,0 +1,13 @@
+// This command line utility provides a way to extract mpq files.
+//
+// Flags:
+// -o [directory] Output directory
+// -v Enable verbose output
+//
+// Usage:
+// First run `go install extract-mpq.go` in this directory.
+// Navigate to the Diablo II directory (ex: C:/Program Files (x86)/Diablo II)
+// then run extract-mpq(.exe) with the filename of the mpq to be extracted.
+//
+// extract-mpq d2char.mpq
+package main

utils/extract-mpq/extract-mpq.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"github.com/OpenDiablo2/OpenDiablo2/d2common/d2interface"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/OpenDiablo2/OpenDiablo2/d2common/d2fileformats/d2mpq"
+)
+
+func main() {
+	var (
+		outPath string
+		verbose bool
+	)
+	flag.StringVar(&outPath, "o", "./output/", "output directory")
+	flag.BoolVar(&verbose, "v", false, "verbose output")
+	flag.Parse()
+
+	if len(flag.Args()) != 1 {
+		fmt.Printf("Usage: %s filename.mpq\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	filename := flag.Arg(0)
+	mpq, err := d2mpq.Load(filename)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	list, err := mpq.GetFileList()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	_, mpqFile := filepath.Split(strings.Replace(filename, "\\", "/", -1))
+	for _, filename := range list {
+		extractFile(mpq, mpqFile, filename, outPath)
+
+		if verbose {
+			fmt.Printf("Writing: %s\n", filename)
+		}
+	}
+}
+
+func extractFile(mpq d2interface.Archive, mpqFile string, filename string, outPath string)  {
+	defer func() {
+		if r := recover(); r != nil {
+			log.Printf("recovered from panic in file: %s, %v", filename, r)
+		}
+	}()
+	dir, file := filepath.Split(strings.Replace(filename, "\\", "/", -1))
+	dir = mpqFile + "/" + dir
+
+	err := os.MkdirAll(outPath+dir, 0755)
+	if err != nil {
+		log.Printf("failed to create directory: %s, %v", outPath+dir, err)
+		return
+	}
+
+	f, err := os.Create(outPath + dir + file)
+	if err != nil {
+		log.Printf("failed to create file: %s, %v", filename, err)
+		return
+	}
+	defer f.Close()
+
+	buf, err := mpq.ReadFile(filename)
+	if err != nil {
+		log.Printf("failed to read file: %s, %v", filename, err)
+		return
+	}
+	f.Write(buf)
+
+}