add a configuration option to export env vars

Add a way to make [Heroku config vars](https://devcenter.heroku.com/articles/config-vars) available at slug compile time.

`elixir_buildpack.config` example:

```bash
# Erlang version
erlang_version=17.0

# Elixir version
elixir_version=(tag v0.14.1)

# Do dependencies have to be built from scratch on every deploy?
always_build_deps=false

# Export heroku config vars
config_vars_to_export=(DATABASE_URL SESSION_SECRET)
```

This remove `export_database_url` and add `export_config_vars`.

Should we keep `export_database_url` for backwards compatibility?