Add ability to use autotls option

[MorphedAU]

Adds the ability to use node-imap's autotls option which is useful for servers that require starttls.

From the node-imap package documentation:

autotls - string - Set to 'always' to always attempt connection upgrades via STARTTLS, 'required' only if upgrading is required, or 'never' to never attempt upgrading. Default: 'never'

[MateMalice]

Thanks for the suggestion - I agree that STARTTLS support is a useful feature. I've merged the request and will publish a patched version of the package.