Add custom hook for customizing output rendering #479

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:

EXPLORER_TRANSFORMS = [
    ("img", '<img src="{0}" />')
]

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:

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

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

def callback(column_name: str, value: Any) -> str:
    if column_name.endswith('_img'):
        return f'<img src="{value}" />'
    if "@" in value:
        return f'<a href="mailto:{value}">{value}</a>'
    if isinstance(value, date) and value < date.today():
        return f'<span style="color:red">{value}</span>'
    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.

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?