Adding an option to control justification in ebooks

If the user includes in `book.json` a configuration like

```
"ebook": {
    "justification": "justify"
}
```

an `--change-justification` option with the configured value will be included when calling calibre's `ebook-convert` tool. 

The call to `ebook-convert` will include something like `--change-justification="justify"`.

If there is no "ebook" or "ebook.justification" option, no `--change-justification` will be included.

The value configured must be one of "original", "left" or "justify". In case of an invalid value, an error is shown.