option to exclude env_vars

We have a handful of sensitive environment variables in each of our apps
(12-factor apps) and we feel it'd be best to just exclude the
environment completely.

In theory this could be done by overriding value_filter with something like:

```ruby
def self.value_filter(value)
  ENV.values.include?(value) ? nil : value
end
```

but this seems a lot more straightforward. Thanks!