Formally add RAW support (including DNG and many others)

Fixes #60

The full list of supported formats and extensions mirrors that of
FreeImage, specifically:

3fr
arw
bay
bmq
cap
cine
cr2
crw
cs1
dc2
dcr
dng
drf
dsc
erf
fff
ia
iiq
k25
kc2
kdc
mdc
mef
mos
mrw
nef
nrw
orf
pef
ptx
pxn
qtk
raf
raw
rdc
rw2
rwz
sr2
srf
sti