add iconv codecs

Summary:
Time time has come for iconv.

```
LookupError: unknown encoding: iso-2022-cn
<ImapFolderSyncMonitor at 0x2cba380> failed with LookupError
```

Test Plan: You can run it on some wonky mail, or just trust me. I've only had this issue with a few messages in my +500k mailbox.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D25