Add Rake task

Allow `haml-lint` to be invoked via Rake task. Example usage:

    require 'haml_lint/rake_task'

    HamlLint::RakeTask.new(:amirite?) do |task|
      task.include_linter = %w[ImplicitDiv RuboCop]
      task.exclude_linter = %w[TagName]
      task.config = File.expand_path("~/.haml_lint.yml")
      task.exclude = %W[#{File.expand_path("some/path.haml")}]
      task.pattern = 'views/**/*.haml'
    end

    task default: :amirite?

Change-Id: I903b08308ef40a1dab6af5ba2c20eb09742240de
Reviewed-on: http://gerrit.causes.com/40774
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Update changelog with rake task addition

Change-Id: Ib339043b7f0995903833c00b5ee38456138be6ec
Reviewed-on: http://gerrit.causes.com/40775
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>