Add beforeSend hook for email customization

- Add BeforeSendHook type and BeforeSendMailOptions interface
- Implement hook execution in MailingService before sending emails
- Hook allows adding attachments, headers, and modifying email options
- Add comprehensive documentation with examples
- Bump version to 0.1.20

src/services/MailingService.ts

@@ -270,7 +270,7 @@ export class MailingService implements IMailingService {
         fromField = this.getDefaultFrom()
       }
 
-      const mailOptions = {
+      let mailOptions: any = {
         from: fromField,
         to: email.to,
         cc: email.cc || undefined,
@@ -281,6 +281,16 @@ export class MailingService implements IMailingService {
         text: email.text || undefined,
       }
 
+      // Call beforeSend hook if configured
+      if (this.config.beforeSend) {
+        try {
+          mailOptions = await this.config.beforeSend(mailOptions, email)
+        } catch (error) {
+          console.error('Error in beforeSend hook:', error)
+          throw new Error(`beforeSend hook failed: ${error instanceof Error ? error.message : 'Unknown error'}`)
+        }
+      }
+
       await this.transporter.sendMail(mailOptions)
 
       await this.payload.update({