Make configuration variables editable.

[gmacdougall]

This moves the configuration variables to be class level attributes with
default values. This allows the values to be modified by an initializer
by the host application.

For example:

Oops::Tasks.additional_paths << 'app/additional_views'
Oops::Tasks.format = 'tar'

[gmacdougall]

This may already be possible, but I couldn't figure out how to do it.

[gmacdougall]

The way to do this is actually through:

# Rakefile
Oops::Tasks.new do |oops|
  oops.prerequisites = 'assets:precompile'
  oops.additional_paths = %w[vendor/something]
end

Closing in favour of that.