Add --new-pairs-days parameter for download-data command.

This parameter allows us to customize a number of days we would like to download for new pairs only. This allows us to achieve efficient data update, downloading all data for new pairs and only missing data for existing pairs. To do that use `freqtrade download-data --new-pairs-days=3650` (not specifying `--days` or `--timerange` causes freqtrade to download only missing data for existing pairs).

freqtrade/configuration/configuration.py

@@ -108,6 +108,8 @@ class Configuration:
 
         self._process_plot_options(config)
 
+        self._process_data_options(config)
+
         # Check if the exchange set by the user is supported
         check_exchange(config, config.get('experimental', {}).get('block_bad_exchanges', True))
 
@@ -399,6 +401,11 @@ class Configuration:
         self._args_to_config(config, argname='dataformat_trades',
                              logstring='Using "{}" to store trades data.')
 
+    def _process_data_options(self, config: Dict[str, Any]) -> None:
+
+        self._args_to_config(config, argname='new_pairs_days',
+                             logstring='Detected --new-pairs-days: {}')
+
     def _process_runmode(self, config: Dict[str, Any]) -> None:
 
         self._args_to_config(config, argname='dry_run',