Move the node-imap configuration to the configuration file

[ChristopheB]

Moving the node-imap configuration to the configuration file:

• lets the user access all the node-imap options
• detaches the code from node-imap, as all the options now reside in the configuration file ; this saves from updating the code every time an option is added or changed

Even though the configuration structure has changed, the code still handles the old structure, for backward compatibility reasons.

[shroux]

^This !

This pull-request could resolve the missing arguments like autotls.