It's hard to create a TextLoader column of type key

It seems that the only way to create a key column for `TextLoader` is using this constructor:

```
public Column(string name, DataKind dataKind, TextLoader.Range[] source, KeyCount keyCount = null)
```

Should we consider adding an optional `KeyCount` argument to the other constructors?

```
public Column(string name, DataKind dataKind, int index);
public Column(string name, DataKind dataKind, int minIndex, int maxIndex);
```