Allow unspecified justification

[FranklinYu]

This is a feature request.

Currently if I had

| abc | def function |
|---|---|
| d | |

It would add default_justification to each column making it

| abc | def function |
|:----|:-------------|
| d   |              |

It would be awesome if we can specify

{
  "default_justification": null
}

so that it doesn’t add justification to any column.