Add user configurable sanitizer fields

Allow user to specify some additional fields to sanitize.  Replaces the values with the string mask (********)

```ruby
Raven.configure do |config|
  config.sanitize_fields = %w(host User-Agent)
end
```
:( passing the sanitize fields config here isn't great, since it's only required by a single, specific processor. I think it's possible to keep the original interface here.