Rake tasks interface

Hopefully this will do the trick of #68.
Essentially it instantiates a `Maid` and wraps the given block in a single rule.
You can even provide dependencies or a description for the `Rake` task as the following example:
```ruby
Maid::Rake::Task.new clean_torrents: [:dependency], description: 'Clean Torrents' do
  trash(dir('~/Downloads/*.torrent'))
end
```