Override default @empty behaviour.

[steveworley]

Describe the bug
We have a clear default favicon that we want to use if none is specified. The = /favicon.ico location doesn't allow us to define a supplementary configuration to handle overriding the favicon.

I understand that this is suitable for 99% of users, if we declared the favicon handler in a separate file we could easily override for our use case and keep the configuration simple for the base.

To Reproduce
Steps to reproduce the behaviour:

1. Add a server include to declare a location for /favicon.ico with a try_files directive to the local
2. Navigate to /favicon.ico

Expected behavior

1. Ability to change behaviour of @empty or location = /favicon.ico