Allow fold kwarg in arrow.get()

Currently there is no way to specify which side of an ambiguous datetime we want when creating with `arrow.get()`. It'd be nice to be able to do the following.

```python
arrow.get("2001-10-28 01:00:00", "YYYY-MM-DD HH:mm:ss", tzinfo="US/Pacific", fold=1)
```

Start by tracing the arrow.get() entry point and how it handles the supplied format and tzinfo arguments. Add support for the shown fold=1 call, and verify that ambiguous datetime creation selects the requested side while existing calls continue to work.

Written by the indexing model from the issue text.