Feature: disable non-standard bindings to standard hotkeys

Some commonly used hotkeys are re-bound. This may be good for users who mainly use codimd but disturbs others who are not used top it. Examples:

* Shift+Crtl+Cursor selects whole words
* in codimd it depends. On text it has normal behaviour, in tables it changes the column alignment
* Shift+Del usually cuts a selected text
* in codimd it cuts the current line

Idea: make an option to enable/disable this behaviour, either globally or individually per user

Start by locating the JavaScript keyboard-binding entry points and the existing user or global preference settings. Verify the Shift+Ctrl+Cursor and Shift+Del examples, then define the option's scope and confirm that disabling it restores standard hotkey behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.