Do not require list for get_configparser().

We can build a sensible list of config file locations on-the-fly.

diceware/config.py

@@ -46,11 +46,13 @@ def valid_locations():
     return result
 
 
-def get_configparser(path_list):
+def get_configparser(path_list=None):
     """Parse `path_list` for config values.
 
     Return a list of paths read and a config parser instance.
     """
+    if path_list is None:
+        path_list = []
     parser = configparser.SafeConfigParser()
     found = parser.read(path_list)
     return found, parser

tests/test_config.py

@@ -30,3 +30,8 @@ class TestConfigModule(object):
         # we cope with empty config file lists
         found, config = get_configparser([])
         assert found == []
+
+    def test_get_configparser_no_list(self):
+        # we cope with no list at all
+        found, config = get_configparser()
+        assert found == []