- open the decoding/encoding procedures to the users
- switch from hardcoded decoding/encoding to pluginable decoders/encoders (builtin + user created)
- `SDWebImageCodersManager` is a singleton holding an array of `SDImageCoder` (protocol). Even though a singleton is not necesarily a good pattern, in this case it eliminates dealing with passing this array around
- uses a priority queue behind scenes, which means the latest added coders have priority.
- the priority is crucial when encoding/decoding something, we go through the list and ask each coder if they can handle the current data (see `canDecodeFromData:`, `canEncodeToFormat:`, `canIncrementallyDecodeFromData:`)
- each coder must conform to this protocol `SDImageCoder` describing all the required behavior for a coder
- we provide 3 built-in coders: `SDWebImageImageIOCoder` (for JPEG, PNG, TIFF), `SDWebImageGIFCoder` (for GIF), `SDWebImageWebPCoder` (for WebP and animated WebP)
- the user of SDWebImage can create custom coders by conforming to `SDImageCoder` and adding the coders to `SDWebImageCodersManager`. See `addCoder:` or `removeCoder:` or `coders` getter to get the array
- in order to preserve backwards compatibility, the UIImage categories were preserved, calling the new coders APIs described above
