config: support comma in config ListAttribute item

This is a first step towards completing sopel-irc#1455, since, as @dgw mentioned,
to do so would require accepting commas in a single entry of ListAttribute.

Items entered through the wizard do not need to be escaped manaully. Items
configured in the `.cfg` file manually require an escaped command, like '\,'.
The prompt has also been changed so that the items are now surrounded by quotes
so that one can differentiate items that may contains commas.

The following scenarios were tested (using the `url.py` module, specifically the
`exclude` setting):

1. Wizard, setting items when there is no default (pre-existing items); mix of w/ commas
and w/o commas
2. Wizard, replacing existing items when a default exists; mix of w/ commas and w/o commas
3. Wizard, extending the current default list; mix of w/ commas and w/o commas
4. Manual edit, setting items; w/ commas (manually escaped) and w/o commas
5. Wizard, setting items w/o default; no commas

sopel/config/types.py

@@ -27,6 +27,7 @@
 from __future__ import unicode_literals, absolute_import, print_function, division
 
 import os.path
+import re
 import sys
 from sopel.tools import get_input
 
@@ -218,16 +219,18 @@ def __init__(self, name, strip=True, default=None):
         self.strip = strip
 
     def parse(self, value):
-        value = list(filter(None, value.split(',')))
+        value = list(filter(None, re.split(r'(?<!\\),', value)))
+        items = map(lambda x: x.replace(r'\,', ','), value)
         if self.strip:
-            return [v.strip() for v in value]
+            return [v.strip() for v in items]
         else:
-            return value
+            return items
 
     def serialize(self, value):
         if not isinstance(value, (list, set)):
             raise ValueError('ListAttribute value must be a list.')
-        return ','.join(value)
+        items = map(lambda x: x.replace(',', r'\,'), value)
+        return ','.join(items)
 
     def configure(self, prompt, default, parent, section_name):
         each_prompt = '?'
@@ -236,17 +239,20 @@ def configure(self, prompt, default, parent, section_name):
             prompt = prompt[0]
 
         if default is not NO_DEFAULT:
-            default = ','.join(default)
-            prompt = '{} [{}]'.format(prompt, default)
+            default_value = self.serialize(default)
+            prompt = '{} [{}]'.format(prompt, ",".join(map(lambda x: '"{}"'.format(x), default)))
         else:
-            default = ''
+            default_value = ''
         print(prompt)
         values = []
-        value = get_input(each_prompt + ' ') or default
+        value = get_input(each_prompt + ' ') or default_value
         while value:
-            values.append(value)
+            if value == default_value:
+                values.extend(self.parse(default_value))
+            else:
+                values.append(value)
             value = get_input(each_prompt + ' ')
-        return self.parse(','.join(values))
+        return self.parse(self.serialize(values))
 
 
 class ChoiceAttribute(BaseValidated):