Add custom hook for customizing output rendering

The `EXPLORER_TRANSFORMS` setting enables the developer to provide a custom template for the values in the given column, for example, to display images if the URL is retrieved from the DB:

```py
EXPLORER_TRANSFORMS = [
("img", '')
]
```

However, this system is not very flexible.
It would be nice to:

* Have the same pattern for all* columns* meeting certain criteria (e.g. ending with `_img`)
* Have the same pattern for all *values* meeting certain criteria (e.g. display as `mailto` link, if it is formated as an email)
* Make use of external variables (e.g. in displaying a deadline show it in red if it is older than the current date)

For enabling such flexibility, I would propose to add the `EXPLORER_DISPLAY_CALLBACK` setting which accepts a function with the following signature:

```py
def callback(column_name: str, value: Any) -> Optional[str]:
pass
```

This would be flexible enough to satisfy all of the aforementioned use-cases:

```py
def callback(column_name: str, value: Any) -> str:
if column_name.endswith('_img'):
return f''
if "@" in value:
return f'{value}'
if isinstance(value, date) and value < date.today():
return f'{value}'
return None

EXPLORER_DISPLAY_CALLBACK = calback

```

Returning `None` would instruct the system to continue on a normal path: check the `EXPLORER_TRANSFORMS` setting and if the column is not present there fall back to displaying the value as usual.

This would mean that the following default implementation would provide 100% backward compatible behavior.

```py
def callback(column_name: str, value: Any) -> Optional[str]:
return None

EXPLORER_DISPLAY_CALLBACK = calback
```

I would be interested in implementing that feature and preparing a PR.

What are your thoughts regarding that new API proposal?