pull meta data with squashfs

[bcumming]

Pull the image meta data (if it exists) when pulling a uenv image.

Currently, the following call downloads a squashfs file from the site registry, and stores it in a path that matches the image hash:

uenv image pull prgenv-gnu

This update also downloads the meta data, and stores it (uncompressed) in a path called meta, alongside the squashfs file.

[simonpintarelli]

Lgtm. Images for which only metadata has been pulled are listed without a note in uenv image ls. It might confuse some users when attempting to run uenv start on an image which only has metadata locally.