Add a "metadata" file contents flag

[justincormack]

Currently this supports "yaml" as the only option, which will output the yaml config (as JSON) into the file specified in the image.

Fix #107

This should be extended to add more metadata, eg moby tool commit and version used to build, and anything else useful.

[deitch]

Can we add a sample use and/or doc?

[justincormack]

AH yes, will update doc; was going to add to linuxkit.yml too

[justincormack]

added doc