HackMD TOC always

With this extension in HackMD, you can see TOC even in edit or split-view mode. The table of contents is displayed in HackMD even in Edit and Split-View modes. It also adds helper buttons to the navigation bar at the top. These features also work in Edit mode. ---- 2021/02 Added a modal dialog for adjusting the TOC, accessible from the navigation bar. 2020/11 Fixed an extraction issue when installing on Windows.