ENH: Support for patching conventions.

This allows iris to handle patching of the conventions global attribute via a defined `cf_patch_conventions` function.
Have you considered making this backwards compatible with configs that don't provide this function or want to make it a null operation? e.g.
```python
cf_patch_conventions = iris.site_configuration.get('cf_patch_conventions')
if cf_patch_conventions is not None:
     conventions = cf_patch_conventions(conventions)
```