More markdown shortcuts

## Problem

Code only has very basic markdown shortcuts.

## Proposal

* Make the backticks \` work like parenthesis: adding a single backtick adds a double and selecting text and inserting a backtick surrounds the text with it. Backtick is used in markdown to create `monospaced text`
* Use `Ctrl`+`Shift`+`1` up to `Ctrl`+`Shift`+`6` for headers. Make it a toggle shortcut.
* `Ctrl`+`Shift`+`P` for inserting images (**p**ictures, get it? ;). Maybe even use a file chooser to select the file.