Add new 'checksums' command, to add sha1 or md5 machine tags.

[samwilson]

Refs #20

[samwilson]

I thought of that, but this seems a separate operation. Do you think it'd be better as part of the download command?

I guess it would then also make sense to check for the existence of checksum tags when downloading, and verify the downloads.

And I think the tag re-adding is wrong here! It probably has to re-add them in their "raw" form, i.e. not these concatenated and lowercase form.

[samwilson]

I've rejigged this to work correctly with adding the new tag.

It's still a separate command, because I think it makes it more flexible this way. We could still add a checksum check to the download command though.

What do you think?