Plotting: add alias --backtest-filename for --export-filename

makes it easier to discover how to use this argument

freqtrade/plot/plotting.py

@@ -61,7 +61,11 @@ def init_plotscript(config, markets: List, startup_candles: int = 0):
                                             startup_candles, min_date)
 
     no_trades = False
-    filename = config.get('exportfilename')
+    for arg_name in ['exportfilename', 'backtestfilename']:
+        filename = config.get(arg_name)
+        if filename is not None:
+            break
+
     if config.get('no_trades', False):
         no_trades = True
     elif config['trade_source'] == 'file':