Spike - Store secondary name for Features

- As a user, I wish Featuretools' Feature objects had the ability to store a secondary name for Features.

Given that multi-output features exist, this ability to store a second name would have to comply with there being multiple output features, and we'd have to decide if we would require secondary names to follow the naming convention of `feature_name[0]`, `feature_name[1]`, or if we would be okay with them not following that convention.

#### Code Example
I'm imagining that the secondary names would exist entirely separately from the primary name logic - since it doesn't get used anywhere in Featuretools itself, it should not be possible to accidentally access the secondary name from any of the existing logic.

```python
feat = ft.Feature(...)
feat.set_secondary_name(...)

name = feat.get_name()
secondary_name = feat.get_secondary_name()
```