add support for regressors in make_future_dataframe

make_future_dataframe doesn't support regressors currently. So code like:

```
m = Prophet()
m.add_regressor('var')
m.fit(df)
forecasts = m.predict(m.make_future_dataframe(periods=7))

```
gives an error like:

`ValueError: Regressor 'var' missing from dataframe when attempting to generate forecasts`

I know prophet may not know what exact values to put for var in each of the rows added but it can create a dataframe with thes(e) column(s) with default values for this.

There is a post on stackoverflow a year ago looking for this: https://stackoverflow.com/questions/64822488/how-to-use-prophets-make-future-dataframe-with-multiple-regressors