Feature request: allow Color Space choices in JpegEncoder

[antonfirsov]

This is a useful image size optimization for users dealing with grayscale images. (See #807)

Additional colorspace encoders to implement:

• Grayscale (actually important)
• Ycck (nice to have)
• Cmyk (nice to have)
• RGB (nice to have)

[brianpopow]

@antonfirsov with #1734 merged, encoding with RGB colorspace is now possible.

[antonfirsov]

@brianpopow nice! Only two more left.

[JimBobSquarePants]

Fixed via #2120