[#139] html_safe config

pull request about issue https://github.com/andi/simple-navigation/issues/139

I think doing 
```ruby
@consider_item_names_as_safe = false
```
by default is kind of weird, because developers write menu code by himself and won't do things like this

```ruby
SimpleNavigation::Configuration.run do |nav|
  nav.items do |top|
    top.item :hack, "<script>alert('hacked!');</script>", '#'
  end
end
```

comments and suggestions about request are welcome!