Add support for plotting time series target data

- As a user, I wish I could use EvalML to quicky plot time series target data.
- In particular, I would like to be able to create a plot of timestamps vs. target data value.
- In addition, I would like to be able to optionally include prediction interval data in this plot.

#### Code Example

```python
pl.plot_time_series_target(X, y)
# returns plotly fig

pl.plot_time_series_target(X, y, plot_prediction_intervals=True)
# returns plotly fig with intervals
```