mender-convert: Compressed input

Raw image files from upstream (Raspbian, BeagleBoard) are generally compressed because this saves significant download time.
Before using these images with mender-convert, the user first has to run a decompression command and wait until it finishes. This can take many minutes and is one more step.

User value (why)

• Save one step (decompress) when converting from upstream image files
• Save several minutes waiting for decompression to complete

Size (SP)

• TBD

Acceptance criteria

• mender-convert supports taking these formats as input: .xz, .gz and .zip in actions 1) from-raw-disk-image (as option --raw-disk-image) and 2) mender-disk-image-to-artifact (as option --mender-disk-image)
• the format is auto-detected (the user does not need to specify which format the image is compressed with)