ENH: Return plot object in Backtest.plot() (#415)

For amending / bokeh.io.export_png

Author: Alexey Peschany

backtesting/backtesting.py

@@ -1706,7 +1706,7 @@ def plot(self, *, results: pd.Series = None, filename=None, plot_width=None,
                 raise RuntimeError('First issue `backtest.run()` to obtain results.')
             results = self._results
 
-        plot(
+        return plot(
             results=results,
             df=self._data,
             indicators=results._strategy._indicators,