Add image references for image/{png,gif,bmp}

Interestingly found the first magic.mime issue not fully being aware of
output from GIMP.

~/code/mimetype/image/bmp> find . -exec file {} \; -exec xxd -l 16 {} \;
reference-16-a1r5g5b5.bmp data
0000000: 424d c6dc 0000 0000 0000 4600 0000 3800  BM........F...8.
reference-16-x1r5g5b5.bmp PC bitmap, Windows 3.x format, 168 x 168 x16
0000000: 424d b6dc 0000 0000 0000 3600 0000 2800  BM........6...(.
reference-32-x8r8g8b8.bmp data
0000000: 424d 46b9 0100 0000 0000 4600 0000 3800  BMF.......F...8.
reference-24-r8g8b8.bmp PC bitmap, Windows 3.x format, 168 x 168 x24
0000000: 424d f64a 0100 0000 0000 3600 0000 2800  BM.J......6...(.
reference-16-r5g6b5.bmp data
0000000: 424d c6dc 0000 0000 0000 4600 0000 3800  BM........F...8.
reference-32-a8r8g8b8.bmp PC bitmap, Windows 3.x format, 168 x 168 x32
0000000: 424d 36b9 0100 0000 0000 3600 0000 2800  BM6.......6...(.