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/book.go

@@ -16,8 +16,7 @@ type Book struct {
 	Container Container `json:"-"`
 	Mimetype  string    `json:"-"`
 	Content   []Content
-
-	fd *zip.ReadCloser
+	fd        *zip.ReadCloser
 }
 
 // Open open resource file
@@ -34,11 +33,6 @@ func (p *Book) Files() []string {
 	return fns
 }
 
-// Close close file reader
-func (p *Book) Close() error {
-	return p.fd.Close()
-}
-
 // -----------------------------------------------------------------------------
 func (p *Book) filename(n string) string {
 	return path.Join(path.Dir(p.Container.Rootfile.Path), n)