Support 2d image segementation use cases

[mbernico]

Is your feature request related to a problem? Please describe.
No, I just want to use TFRecorder on a different problem type.

Describe the solution you'd like
As a user I want to be able to use TFRecorder for image segmentation use cases.

In these my input data consists of an image / mask tuple where the mask is a 2-d numpy array with dimensions image_height x image_width. Each pixel in the mask contains an int label, labeling the type of that pixel.