Quick and dirty addition of title "Table of Contents"

[AlfonsoRReyes]

This is a quick and dirty addition of a title to the TOC plugin, like in Typora.

This how it the TOC block will look after the modification:

Instructions

1. Modify line 108. Instead of const use let.
2. Modify line 111. Add a level 2 heading as the title "Table of Contents" to the variable markdown.

Like this:

[johansatge]

👍 see #5