RFC: Adding marc tags to field descriptions

Currently, the schema for Marc21 contain the "human readable marc tag". E.g. ""title_statement" gives

```
"description": "Title Statement"
```

It would be helpful if we could also hold the numeric tag, ie. `245`.
- This serves as sort of a unique identifier
- It helps inline documentation
- It is easy to construct a lookup url form that (e.g. http://www.loc.gov/marc/bibliographic/bd245.html), that can serve well in a help functionality.
- It might serve in some upcoming editor for a "non-human" format (being usually preferred by full time cataloguers due to it's compactness).

Without changing the model one could just add that to "description". Similarly, it might be very helpul to have the subfield codes in some sort of description as well (due to their uniqueness). E.g.

```
"title_statement": {
"description": "Title Statement",
"tag": "245",
"title": {
"type": "string",
"subfield" : "a"
},
```