feat: support email SendHtml (#30)

Signed-off-by: Ryota Sakamoto <sakamo.ryota+github@gmail.com>

docs/services/email.md

@@ -9,6 +9,7 @@ The Email notification service sends email notifications using SMTP protocol and
 * `username` - username
 * `password` - password
 * `from` - from email address
+* `html` - optional bool, true or false
 
 ## Example
 

pkg/services/email.go

@@ -56,6 +56,7 @@ type EmailOptions struct {
 	Username           string `json:"username"`
 	Password           string `json:"password"`
 	From               string `json:"from"`
+	Html               bool   `json:"html"`
 }
 
 type emailService struct {
@@ -73,12 +74,18 @@ func (s *emailService) Send(notification Notification, dest Destination) error {
 		subject = notification.Email.Subject
 		body = text.Coalesce(notification.Email.Body, body)
 	}
-	return smtp.New(smtp.Options{
+	email := smtp.New(smtp.Options{
 		From:               s.opts.From,
 		Host:               s.opts.Host,
 		Port:               s.opts.Port,
 		InsecureSkipVerify: s.opts.InsecureSkipVerify,
 		Password:           s.opts.Password,
 		Username:           s.opts.Username,
-	}).WithSubject(subject).WithBody(body).To(dest.Recipient).Send()
+	}).WithSubject(subject).WithBody(body).To(dest.Recipient)
+
+	if s.opts.Html {
+		return email.SendHtml()
+	} else {
+		return email.Send()
+	}
 }