Support -w and -h flags

These set the desired width and height of the thumbnail, respectively.

main.go

@@ -2,16 +2,21 @@ package main
 
 import (
 	"code.as/captureas/go-captureas"
+	"flag"
 	"fmt"
 	"os"
 )
 
 func main() {
-	if len(os.Args) < 2 {
+	w := flag.Int("w", 800, "Desired width of the screenshot")
+	h := flag.Int("h", 600, "Desired height of the screenshot")
+	flag.Parse()
+
+	if len(flag.Args()) < 1 {
 		fmt.Println("usage: cap [url]")
 		os.Exit(1)
 	}
-	url := os.Args[1]
+	url := flag.Args()[0]
 
 	username := os.Getenv("CAPTUREAS_USER")
 	secret := os.Getenv("CAPTUREAS_SECRET")
@@ -28,7 +33,7 @@ func main() {
 	c := captureas.NewClient(username, secret)
 	fmt.Println(c.Capture(&captureas.CaptureParams{
 		URL:    url,
-		Width:  800,
-		Height: 600,
+		Width:  *w,
+		Height: *h,
 	}))
 }