Simplify implementations of commands

Trying to make it less magic. We don't really need to preload the
things that we're preloading, and I want to avoid unexpected side
effects.
Remove command preload lists feature

We're not using it internally any more, and it's easy to achieve without
this specific way of doing it, so just explain that in the README.
Set ARGV by default

Rather than every command having to do it
Use the actual bin file for a command where possible
Refactor rails commands
Organise commands into subdirectories
Seed the random number generator on each run

Fixes #113