Make the middleware more extensible

[browniebroke]

I have a similar issue as what was raised in #5 and was thinking of adding a LOCKDOWN_HOST_EXCEPTIONS. As the idea was previously rejected, I wanted to try to subclass the LockdownMiddleware and add my own logic before main one, but found that this was a bit more difficult that I thought.

Basically, I was thinking of adding my logic after the initial checks, and before further checks are done:

django-lockdown/lockdown/middleware.py

Lines 92 to 94 in be8224e

	# Don't lock down if django-lockdown is disabled altogether.
	if getattr(settings, 'LOCKDOWN_ENABLED', True) is False:
	return None

I think that could be achieved by calling a method on the middleware class which by default wouldn't do anything, but which would enable user to hook into their custom logic by returning something specific.

Changed middleware:

class LockdownMiddleware(object):
    ...
    def process_request(self, request):
        ...
        # Don't lock down if django-lockdown is disabled altogether.
        if getattr(settings, 'LOCKDOWN_ENABLED', True) is False:
            return None

        # NEW: Call hook
        if self.is_request_excluded(request):
            return None

        ...

    def is_request_excluded(self, request):  # New method
        """Hook for users to implement a custom logic to exclude the request."""
        return False

Example implementation in user-land:

from lockdown.middleware import LockdownMiddleware as BaseLockdownMiddleware


class LockdownMiddleware(BaseLockdownMiddleware):
    def is_request_excluded(self, request, response):
        return request.get_host() in ["api.mysite.com"]

What do you think?