Move this into settings/base.py

Perhaps, instead, *copy* it into settings/base.py (as `SEARCH_MIDDLEWARE_IMPL = None`) and change this line to be: 
```
# Specify the middleware implementation to use in the middleware 
#SEARCH_MIDDLEWARE_IMPL = 'elasticsearch'  # for example
```
That way it becomes dead obvious how you're supposed to override it if need be.