Add `retry` parameter to http-ish storages

I use vdirsyncer with cron to backup caldav and carddav data from posteo.de

At one of two sync actions, I get at least one "connection refused" error.

error: Unknown error occured for my_calendar/jtmfam: HTTPSConnectionPool(host='posteo.de', port=8443): Max retries exceeded with url: /calendars/its.me/abc/ (Caused by NewConnectionError('<requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x71797070>: Failed to establish a new connection: [Errno 111] Connection refused',))
error: Use `-vdebug` to see the full traceback.
error: Unknown error occured for my_calendar/default: HTTPSConnectionPool(host='posteo.de', port=8443): Max retries exceeded with url: /calendars/its.me/default/ (Caused by NewConnectionError('<requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x717dbd10>: Failed to establish a new connection: [Errno 111] Connection refused',))
error: Use `-vdebug` to see the full traceback.
error: 2 out of 12 tasks failed.

Is there a possibility to allow more than one retry?
Thanks a lot for this great tool!

No file or test is named in the issue. Start by locating the http-ish storage implementation and its connection or retry handling, then inspect existing storage configuration tests; done means users can configure more than one retry and the behavior is covered by tests.