add image/default as default feature

[cospectrum]

Continuing our discussion about the re-export of the image #557.

image/default should allow to run examples using imageproc::image, otherwise a panic/error may occur (png/jpeg decode, etc).
It seems to me that users will expect this from our re-export.
If they don't need the default image features, then they will do imageproc = { ..., default-features = false } which will lead to image = { ..., default-features = false } internally.

The msrv is increased, see image-rs/image#2168

[theotherphil]

Thanks! Let me know if there’s anything you think needs to go in before the next release, otherwise I’ll just ship what we’ve got around 12 hours from now.