fix: add support for STARTTLS

[andrinoff]

Fixes #217

Add STARTTLS and InsecureSkipVerify support for local proxies

Changes:

1. config/config.go

   • Added an Insecure boolean flag to the Account and rawAccount structs.

   • Updated LoadConfig JSON unmarshaling to parse and map the "insecure" field from the configuration file.

2. fetcher/fetcher.go

   • Modified the connect() function to check the configured IMAP port.

   • Added fallback to establish a standard TCP connection (client.Dial) and upgrade it via StartTLS when standard unencrypted ports (1143, 143) are used, rather than forcing implicit TLS (client.DialTLS).

   • Injected tls.Config{InsecureSkipVerify: account.Insecure} into both connection methods to bypass strict certificate validation when the flag is enabled.

3. sender/sender.go

   • Replaced the use of smtp.SendMail with a manually unrolled SMTP transaction (Dial, Hello, StartTLS, Auth, Data, Quit).

This allows the injection of a custom tls.Config with InsecureSkipVerify during the STARTTLS phase, which is not supported by the default smtp.SendMail implementation.