Add option to specify output file type

[jcreinhold]

🚀 Feature

Add an option in the CLI to specify the output file type (e.g., NIfTI).

Motivation

The current implementation of the CLI saves the output image as the same type as the input; however, a user who inputs DICOM might want NIfTI to be returned, especially because the DICOM won't be saved as single-slice frames (it'll be saved as a multi-slice frame).