Added simple HTML output

I wanted reek to output as html file. So I added it.

`reek -H . `

or 

`reek --html .`

will output a `reek.html` file with all errors and messages
The code itself looks good but it kind of "feels" wrong since there is so much logic here.
What do you think about using something like:

```Ruby
require 'erubis'

class HtmlReport < Report
  TEMPLATE = File.read(File.expand_path("../../../../assets/html_output.html.erb", __FILE__))
  # ....
end
```

and then using an instance of this here?