Update section of summarize can print secrets in plain text

[enicholson125]

We use tftools summarize as part of our CI flow because it is fantastic and considerably simplifies reviewing Terraform changes. However we've found that the diffs added into the Update section in 0.9.0 can print secrets in plain text. An example of the output we've seen is:

🟡 Update:
⚠️ module.this.module.db.module.db_instance.aws_db_instance.this[0]
password: OhNoMyPassword

It would be great if tftools hid values marked as sensitive.