Update email imap segment to allow ENV vars usage for credentials

This pull request fixes the issue #804 

It allows the user to load its credentials from environment variables instead of the json file. Why ? Because I use almost the same dotfiles between my personal and my professional computers. So I had  to duplicate the whole powerline theme just for the email imap segment. 

I think a good alternative could be to load the credentials from the environment, if not defined in the json file. What do you think about that ?