Consistency in named arguments

[supakeen]

We have a few arguments that all deal with filesystem paths:

1. --blueprint
2. --datadir
3. --output-dir

These either have dir, -dir, or nothing special in the name. I'd like to make them predictable and my suggestion is to drop the dir and -dir parts of the names. So we get:

1. --blueprint
2. --data
3. --output

Thoughts? --data might be a bit terse and in general is pretty non-descriptive; perhaps --defs suits better to align with the name that seems to be the one we want in images?

--output clashes with the --output on list-images, I'd suggest renaming that one to --format.

A full command line would then look like (for example):

image-builder build --defs=~/my-defs --blueprint=foo.toml --output=/opt/build qcow2

[supakeen]

Demo implementation in #99 so it can be tried out to see how it feels; each change as a separate commit so they can be cherry-picked for those we like.