Create Issue class

WIP

With the Issue class we can sort issues by path or description. We should provide an option `order` (path or description). Sorting by description can help identify same issue in different files. For example a missing alt attribute in the footer on every page.
OOC, why not

```ruby
[
  'checkable',
  'checks',
  'issue'
].each { |c| require File.join(File.dirname(__FILE__), 'proofer', c) }
```

?