Add a Rake task for use with... Rake

Example:

```ruby
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("terrible_syntax/goaway.haml")}]
  task.pattern = 'views/**/*.haml'
end

task default: :amirite?
```

Note: I didn't write tests for this because RuboCop didn't appear to have tests for their RakeTask class. If @causes would like tests for the RakeTask class, I'm happy to write them.