Add nb_hdrs to fast_app

[Isaac-Flath]

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name: nb_hdrs in fast_app
about: nb_hdrs arg from FastHTML missing from `fast_app
title: '[PR] '
labels: ''
assignees: ''

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Proposed Changes
The FastHTML class has nb_hdrs argument which allows you to opt out of headers being injecting into notebook dom. This PR adds that to fast_app as well so the option exists there as well.

Types of changes
What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist
Go over all the following points, and put an x in all the boxes that apply:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I am aware that this is an nbdev project, and I have edited, cleaned, and synced the source notebooks instead of editing .py or .md files directly.