Add placeholders for Title and PreHeaderText

[b0rt]

for now 'only' some placeholders are available

https://sendportal.io/docs/v2/features/templates

    {{email}} – The subscriber's email
    {{first_name}} – The subscriber's first name (if set)
    {{last_name}} – The subscriber's last name (if set)
    {{unsubscribe_url}} – A URL generated to allow the subscriber to unsubscribe
    {{webview_url}} – A URL generated to allow the subscriber to view the message in a web view

as per my reserach a lot of email templates contain sections like

• title
• preheader-text

Pre Header

Email preheader is the summary text that follows a subject line when the email is viewed in an inbox. In many cases, it’s used to provide a short summary of the email, and is typically one sentence long.

https://www.campaignmonitor.com/resources/glossary/email-preheader/

title

Is it useful to have a <title> in the email templat?

only for 'view in browser' as per this litmus thread

[JonoB]

Happy to accept a PR for this!

[HeadStudios]

Would be great for any references to the code where placeholders are processed so I could create a custom placeholder for prospect's email (so I could use it in a custom link for a prospect to 'opt-in')