fix XML parsing (#2)

- Use antchfx/xmlquery for easier XML parsing. No more regexp wrangling and the result is much more reliable over a variety of ebooks. Much good.
- fix chapter selection, look for `<?xml[...]` which is much more reliable
- add option `-x` to dump the XML ebook source for debugging

pkg/epub/epub_test.go

@@ -1,36 +1,22 @@
 package epub
 
 import (
-	"log"
 	"testing"
 )
 
 func TestEpub(t *testing.T) {
-	bk, err := open(t, "test.epub")
+	_, err := open(t, "test.epub")
 	if err != nil {
 		t.Fatal(err)
 	}
-
-	defer func() {
-		if err := bk.Close(); err != nil {
-			log.Fatal(err)
-		}
-	}()
-
 }
 
 func open(t *testing.T, f string) (*Book, error) {
-	bk, err := Open(f)
+	bk, err := Open(f, false)
 	if err != nil {
 		return nil, err
 	}
 
-	defer func() {
-		if err := bk.Close(); err != nil {
-			log.Fatal(err)
-		}
-	}()
-
 	t.Logf("files: %+v", bk.Files())
 	t.Logf("book: %+v", bk)