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/ncx.go

@@ -7,8 +7,9 @@ type Ncx struct {
 
 // NavPoint nav point
 type NavPoint struct {
-	Text   string     `xml:"navLabel>text" json:"text"`
-	Points []NavPoint `xml:"navPoint" json:"points"`
+	Text    string     `xml:"navLabel>text" json:"text"`
+	Content Content    `xml:"content" json:"content"`
+	Points  []NavPoint `xml:"navPoint" json:"points"`
 }
 
 type Title struct {