ENH: Add `into` argument for `to_xarray` method? 
#### Is your feature request related to a problem?

I would like to able to specify the object used for creating `xarray` objects.  

#### Describe the solution you'd like
Right now, the only option is `xr.Dataset` but I tend to prefer `xr.DataArray` objects.

#### API breaking implications

It shouldn't affect the API in any complicated ways. 

#### Describe alternatives you've considered

Create a `xr.Dataset` and then convert that to `xr.DataArray`.  However, this step could be obviated by usage similar to [`to_dict`](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_dict.html) method.
#### Additional context


```python
import pandas as pd
import xarray as xr
df = pd.read_csv("https://pastebin.com/raw/dR59vTD4", sep="\t", index_col=0)
da = df.to_xarray(into=xr.DataArray)
```
