DIRENV_* variables are reseted when you cd to directory with .envrc

Hello,
I encountered the problem with setting special DIRENV_* variables.
They're reseted when you cd to directory with .envrc.

How to reproduce:
```
$ cd ~
~
$ export DIRENV_CONFIG=~/.direnv; env | grep DIRENV_CONFIG
DIRENV_CONFIG=/Users/ardecvz/.direnv
$ cd ~/dir_with_envrc
.envrc is blocked from loading. Run `direnv allow` to approve it's content for loading.
$ env | grep DIRENV_CONFIG
# Nothing, but it should output DIRENV_CONFIG=/Users/ardecvz/.direnv
$ cd ~
direnv: unloading
$ env | grep DIRENV_CONFIG
# Nothing, but it should output DIRENV_CONFIG=/Users/ardecvz/.direnv again
```

So direnv resets previously set variables. It doesn't matter if you execute direnv allow or not.
As I understand, these variables (DIRENV_CONFIG, DIRENV_BASH and so on) exist to tune your installation so it should be kept to the new env created by direnv.

I have zero knowledge of Go but you should pay attention to this code in env.go:92
```go
func (env Env) Filtered() Env {
	newEnv := make(Env)

	for key, value := range env {
		if !ignoredKey(key) && !direnvVar(key) {
			newEnv[key] = value
		}
	}

	return newEnv
}
```
It filters _ALL_ service variables. There's no separation of user defined and internal DIRENV_* variables.

As a quick fix to keep special DIRENV_* variables we may add them to the ignore list.