Warn us when token strings (e.g. {{ or }}) exist in Preheader.

[Shintaro_Kaketa]

We can input tokens in Preheader, however these tokens do not work correctly in some cases.
Tokens aren't supported in the WYSIWYG preheader, so please consider to warn us when there are token strings (e.g. {{ or }} ) in Preherder.

Reference
https://nation.marketo.com/t5/Product-Discussions/Problem-with-Lead-First-Name-Token-in-an-email-Pre...

https://docs.marketo.com/display/public/DOCS/Email+Editor+v2.0+Overview
Tokens do not work in the Preheader when using the email editor.

[Shintaro_Kaketa]

We can see a warning message (Lead/Company tokens can be used in Pre-headers only with Trigger Campaigns.) at the current release.  Excellent!