ropensci/ijtiff: Add `read_tags()` and `count_imgs()`

NEW FEATURES

• count_imgs() counts the number of images in a TIFF file without reading the images themselves.
• read_tags() reads the tags from TIFF images without reading the images themselves.

• Now includes citation information.
• C code is more readable.
• display() is more flexible, accepting 3 and 4-dimensional arrays, just displaying the first frame from the first channel.