Explicitly require `oscar` in template Vagrantfile

Since the templated Vagrantfile has to reference the `Oscar` constant,
in order to read the Vagrantfile we have to require the oscar plugin.
Vagrant will normally load all plugins before reading the Vagrantfile,
but using the `vagrant plugin` subcommands will _not_ load plugins but
_will_ read the Vagrantfile. To avoid evaluating an uninitialized
constant we need to explicitly require this constant.

It's up for grabs whether using `Vagrant.require_plugin` is actually
better than `require`, but we'll have to see.
[command/init_vms] Add -h flag
[command/init_vms] simplify vm list generation
[command/init_vms] extract actual config writing
[command/init] Add -h flag
Merge branch 'init-add_help'
Merge branch 'refactor-init_vms'
Merge branch 'require-oscar'
Update README for oscar 0.3.0+
Merge branch 'update-readme'