Save image with specified format

[Bruflot]

Ran into the use case where I wanted to save an image without specifying the file extension in the filename. This PR lets you do just that:

img.save_with_format(&"path/to/img_file", ImageFormat::PNG);

I'm not sure if this is a wanted addition or not, so I'm mostly looking for feedback.

I used the image::ImageFormat enum for accepting formats. This enum doesn't include pbm, pgm, ppm, and pam, so I left those out as I'm not familiar enough with the library to make changes to the enum. Alternatively we can just make another enum.

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[HeroicKatora]

Code itself looks very neat overall.

[HeroicKatora]

This enum doesn't include pbm, pgm, ppm, and pam, so I left those out as I'm not familiar enough with the library to make changes to the enum.

The PNM variant uses pam for all encoders, since the format is flexible enough to be basically a superset of the other netbpm formats. Nothing to worry about. It's not the most used format (without having any stats for it but the encoder didn't exist for 5 years 😉 ) and can be added afterwards if required.

[HeroicKatora]

I'm not sure if this is a wanted addition or not, so I'm mostly looking for feedback.

Just to be clear. YES! Definitely very much appreciated. This is also the first and a necessary step towards #982 . And it addresses #834 partially. Just making a note here, so that we don't forget to mark it.

[Bruflot]

Thanks for the feedback! 😄 I made the changes to the docs, and exposed the save_buffer_with_format function in lib.rs (I forgot to do that earlier).

[HeroicKatora]

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