Add missing EXR and support HDR (RGBE)

[iwatakeshi]

The Exr format is defined in the Type enum but was not implemented. This adds that missing implementation. It also adds support for .hdr files (RGBE).

Note that a few online image converters such as online-convert uses a different file signature for .hdr files than the one defined by Wikipedia. This PR covers both cases.

[svartalf]

Yeah, you are right, somehow I forgot to add the .exr implementation back in the days.

By any chance you are going to do some more work with this library? I'm more than welcoming this, but it would be very exhaustive to me to publish a new cargo version after each merge :)
If you are going to implement some more stuff, I was thinking about accumulating all the changes and publishing as one version.

[iwatakeshi]

@svartalf This should be the last change... 😅. I just ran some tests and all passed so we're good now. Thanks.