create vendors module to support idiosyncratic IMAP providers

[zevaverbach]

This was alluded to in #123. Here are a few things such a module could support:

• map flag names to standardized folder names, and vice versa. For example, in Gmail the archived messages correspond with "[Gmail]/All Mail" (including the double quotes!), whereas in Yahoo Mail it's Archive
• when there's an authentication error when instantiating Imbox, helpful error messages might be displayed. For example, with Yahoo: Have you clicked "Allow apps that use less secure sign in" here?: https://login.yahoo.com/account/security.
• support IMAP extensions, which at least Gmail uses. (also alluded to in support Gmail's IMAP extensions #123)

Would you accept a PR that accomplishes this, at least for a few large IMAP providers?

[martinrusev]

@zevaverbach a vendors module would be an awesome addition. I would gladly accept a PR.

[zevaverbach]

Will do! Would it be acceptable to employ such a module “magically“ in the constructor of Imbox? In addition to being able to do so manually .

[martinrusev]

@zevaverbach What I think would work well:

# vendors/gmail
GmailMessages(Messages):
   ..... gmail specific logic

# vendors/__init__.py
__all__ = [GmailMessages]

with Imbox('imap.gmail.com',
        username='username',
        password='password', 
vendor="gmail"):

(or) the magic option

with Imbox('imap.gmail.com) -> parse the hostname, domain is `gmail`, switch to vendor `gmail` automatically 

https://github.com/martinrusev/imbox/blob/master/imbox/__init__.py#L68

from imbox.vendors import GmailMessages

# Set the default
messages_class = Messages
if vendor == "gmail":
  messages_class = GmailMessages

return messages_class(connection=self.connection,
                        parser_policy=self.parser_policy,
                        **kwargs)

[zevaverbach]

@martinrusev super elegant, thank you. Will submit a PR soon.