Add human-friendly size support

Allow sizes to be specified in terms of K, M, G, or T.

Internally, mkvm will convert all input to Kibibytes and then convert these to the expected unit size as needed.  RAM is defined in terms of Mebibytes, disks in terms of Kibibytes.

We also drop the 'tiny' VM template.  'small' is as small as we go now.

Update the README to reflect these changes.

Finally, added a few `opts.separator` elements to make the help output a little easier to read.