Nested If Cells

This enables the user to rename the `__test__` input tendril of an `If` cell. There are two benefits:

* You apply a more meaningful (human digestable) name to the conditional tendril of an if cell
* You can nest these cells, effectively applying an `AND` across all if conditional tendrils. i.e.

```
if __test1__ AND __test2__ : process mycell
```

An example is in the included test.