Default-driven option handling

[andymeneely]

The Ruby named parameters thing doesn't handle so well when we have options that override other options as in layouts. So, let's go back to using hashes in all of our API commands, but implement a more complex set of defaults

The precedence order goes like this:

1. System-wide defaults, such as :x => 0.
2. Command-level defaults override system-wide. e.g. `:color => :black' for text
3. config.yml settings, wherever we support those.
4. Settings specified by the commands themselves, e.g.text 'blah', x: 5, color: :blue

This means we'll need to change away from Ruby parameters and make use of hash merges along with input handlers that just modify the parameters.