feat: Support inline image(BI)

[kvii]

Close: #30

Saving image data as string because I don't want to break current API.
Add Value.RawBytes for getting image data conveniently.

An example of extracting inline image from 99.pdf:

bb, _ := os.ReadFile("99.pdf")
br := bytes.NewReader(bb)
pr, _ := pdf.NewReader(br, br.Size())

var i int
contents := pr.Page(1).V.Key("Contents")
pdf.Interpret(contents, func(stk *pdf.Stack, op string) {
	if op != "BI" {
		return
	}

	bi := stk.Pop()
	w := int(bi.Key("W").Int64()) // 95
	h := int(bi.Key("H").Int64()) // 96
	// decode Image Data
	id := bi.Key("ID").RawBytes()
	br := bytes.NewReader(id)
	rd := ccitt.NewReader(br, ccitt.MSB, ccitt.Group4, w, h, nil)
	bs, _ := io.ReadAll(rd)

	// write image
	img := image.NewGray(image.Rect(0, 0, w, h))
	for y := range h {
		for x := range w {
			idx := y*((w+7)/8) + x/8
			if idx >= len(bs) {
				break
			}
			bitIdx := 7 - (x % 8)
			bit := (bs[idx] >> bitIdx) & 1
			img.SetGray(x, y, color.Gray{Y: bit * 255}) // 0->0, 1->255
		}
	}
	var buf bytes.Buffer
	err = png.Encode(&buf, img)
	if err != nil {
		t.Fatal(err)
	}
	out := buf.Bytes()

	i++
	os.WriteFile(fmt.Sprintf("out%d.png", i), out, 0666)
})