Issue 26: @datatype / @type on columns

This is a request for comments on Issue 26
<https://github.com/w3c/csvw/issues/26> re Metadata Vocabulary for Tabular
Data:

https://github.com/w3c/csvw/issues/26

Proposal: allow additional @datatype or @type whose value would be a RDF
class specifying the type of data in that column.

```
schema: {
  columns: [
   {
     ...
     @(data)type: some-uri ...
   }
  ]
}
```

Rufus