Add null_value metadata to fields

## Background
Sometimes `null` is represented in a column in a way that cannot be covered by `--null-value` command-line option, either because globally cause mess in data (e.g. "0' used as undefined date) or not available in the _format_.

## Feature
Make a column metadata entry `null_value` a string that in src/raw represent `null` for standardization.

## Example
metatada `null_value` = _NOTHING_
| Raw |
| --- |
| Hello |
| Nothing |
| NOTHING |
| World |

| Standardized |
| --- |
| Hello |
| Nothing |
| `NULL` |
| World |