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/commands/cli_options.py

@@ -345,6 +345,13 @@ AVAILABLE_CLI_OPTIONS = {
         type=check_int_positive,
         metavar='INT',
     ),
+    "new_pairs_days": Arg(
+        '--new-pairs-days',
+        help='Download data of new pairs for given number of days. Default: `%(default)s`.',
+        type=check_int_positive,
+        metavar='INT',
+        default=30,
+    ),
     "download_trades": Arg(
         '--dl-trades',
         help='Download trades instead of OHLCV data. The bot will resample trades to the '