Added new option - url_file_to_attachement

Currently, if message contain shared file, slack-backup will try to
download it. If it fail, than empty file will remain, which will be at
least confusing. This will mostly happen for shares which are not
uploaded to the slack servers.

New option will be used to indicate if slack-backup should convert such
share as an attachment, or to save the list of URL and their
destination in local file system to be download manually by the user.

slack_backup/config.py

@@ -13,8 +13,10 @@ class Config(object):
     """Configuration keeper"""
 
     ints = ['verbose', 'quiet']
+    bools = ['url_file_to_attachement']
 
-    sections = {'common': ['channels', 'database', 'quiet', 'verbose'],
+    sections = {'common': ['channels', 'database', 'quiet', 'verbose',
+                           'url_file_to_attachement'],
                 'fetch': ['user', 'password', 'team', 'token'],
                 'generate': ['output', 'format', 'theme']}
 
@@ -35,7 +37,8 @@ class Config(object):
                          'token': None,
                          'output': None,
                          'format': None,
-                         'theme': None}
+                         'theme': None,
+                         'url_file_to_attachement': False}
         # This message supposed to be displayed in INFO level. During the time
         # of running the code where it should be displayed there is no
         # complete information about logging level. Displaying message is
@@ -79,6 +82,8 @@ class Config(object):
             for option in self.sections[section]:
                 if option in self.ints:
                     val = self.cp.getint(section, option, fallback=0)
+                elif option in self.bools:
+                    val = self.cp.getboolean(section, option, fallback=False)
                 elif option == 'channels':
                     val = self.cp.get(section, option, fallback='[]')
                     val = json.loads(val)