Add ability to provide default JavaScript load paths

This allows us to add our own load paths and thus load external JavaScript files such as custom less functions. For example we could do the following, note for this to work the gem must not be required by the Gemfile due to auto initialisation of the loader.

```ruby
  module Less
    module Defaults
      def self.extended(base)
        base.javascript_paths << ::Rails.root.join('lib', 'assets', 'javascripts')
      end
    end

    module Customisations
      def self.included(base)
        loader = base.instance_variable_get :@loader
        loader.require('less/custom-less-functions')
      end
    end
  end

  require 'less'

  module Less
    include Less::Customisations
  end