Support SMTPS when using report -to-email

[shopper29]

What did you implement:

The purpose of this pull request is to provide support for SMTPS when use report -to-email.

Fixes #945

Type of change

• New feature (non-breaking change which adds functionality)

Key points of changes

【report/email.go】
When vuls report -to-email using port465(SMTPS), smtp client use TLS connection.

How to use SMTPS

Setting config.toml to use port465

[email]
smtpAddr      = "smtp.xxx.xxx.com"
smtpPort      = "465"
user          = "xxxx"
password      = "xxxxxxx"
from          = "[email protected]"
to            = ["[email protected]"]
subjectPrefix = "[vuls]" 

Reference

• https://golang.org/pkg/net/smtp/
• https://gist.github.com/chrisgillis/10888032

[kotakanbe]

LGTM