Reduce permissions scope

[evictor]

There is no need for this plugin to "read and change data on all [a user's visited] web sites." At the very least, it should be limited to the web sites it supports, which if I'm not mistaken is a half dozen or so known domains.

[adam-p]

Except that MDH works in a surprising number (and variable number) of places, so it's not possible to enumerate them. See:
https://github.com/adam-p/markdown-here/wiki/Compatibility

However, I keep meaning to look into whether there's some mechanism where the extension can request permissions to a site when the user clicks the button. It's not like MDH needs access before then. So I'm leaving this issue open.