Implement LOGIN auth

[jerbob92]

I recently came across an SMTP server that supported LOGIN auth but not PLAIN.

LOGIN and PLAIN are almost the same, PLAIN just sends everything in 1 command, LOGIN sends seperate commands for the auth/username/password, so I decided to implement it in the same auth logic.

[xhit]

Thanks for pull request!

I'm going to review the change in 24 hours or least because I'm traveling.

[xhit]

In version 2.3.0 was added this feature, also CRAM-MD5 Authentication.

For this, in SMTPServer struct obtained by NewSMTPClient() function, the Authentication variable should be an authType: AuthPlain, AuthLogin, AuthCRAMMD5

If was not specified, default is AuthPlain.

Example:
server.Authentication = mail.AuthLogin

Thanks!, your code help me with the LOGIN Auth.

Also Go Modules compatibility was added.

Thanks for pull request!