[Enhancement] Add Configuration for TOC helper to control header levels

[BradleyGoulding]

Is your feature request related to a problem? Please describe.
The TOC shortcode plugin assumes a default configuration of 4 header levels.

Describe the solution you'd like
I would like a basic configuration option to set which header levels to apply to the TOC

Describe alternatives you've considered
Rework documentation to limit what headers are used, however this is less than ideal when using sub headings for clarity but not navigation

Additional context
Screenshot showing content with only h2 in the TOC

[estruyf]

Your change has been included in version 1.12.0 and got released.