Identify thumbnail offset and length tags by name

[drewnoakes]

After recent changes, these two tags are displayed as unknown.

(From #262):

[Exif SubIFD] - New Subfile Type = Reduced-resolution image
[Exif SubIFD] - Compression = JPEG (old-style)
[Exif SubIFD] - X Resolution = 300 dots per inch
[Exif SubIFD] - Y Resolution = 300 dots per inch
[Exif SubIFD] - Resolution Unit = Inch
[Exif SubIFD] - Unknown tag (0x0201) = 1080832
[Exif SubIFD] - Unknown tag (0x0202) = 1737279
[Exif SubIFD] - YCbCr Positioning = Datum point

0x0201 is thumbnail offset, and 0x0202 is thumbnail length.