Support more headers (H5, H6)
The currently supported headers are H2,H3,H4.
The H1 is not take in account because it is the title of the page.
H2,H3 are headers allowed to be created from the text editor Web Part of SharePoint.
Additional headers can be created by using the Markdown Web Part
Because some additional headers level can be created with the Markdown Web Part, they should be take in account by the TOC extension too.