Trying out (in Feature2) a different approach to the Feature schema.

@laserson, thoughts?
@laserson RE: null, gotcha. Makes sense. Might also be useful for boolean-style tags (e.g., SOMATIC in VCF)...

If you want a fast (but uglier) sol'n, you might do:

```
union { null, string } stringVal = null;
union { null, long } longVal = null;
union { null, double } doubleVal = null;
```

It'd be faster and _might_ compress better? I'm not sure how RLE/etc. works on repeated columns in Parquet...