Make backtesting report markdown shareable

Small tweak to make the backtesting report markdown ready and much easier to share reports on many markdown publishing tools and editors that already support Markdown Extra with just a copy and paste

Example:
![Example](https://i.imgur.com/HXlNkfm.png)

freqtrade/optimize/backtesting.py

@@ -106,7 +106,7 @@ class Backtesting(object):
             len(results[results.profit_BTC > 0]),
             len(results[results.profit_BTC < 0])
         ])
-        return tabulate(tabular_data, headers=headers, floatfmt=floatfmt)
+        return tabulate(tabular_data, headers=headers, floatfmt=floatfmt, tablefmt="pipe")
 
     def _get_sell_trade_entry(
             self, pair: str, buy_row: DataFrame,