Allow users to toggle off the default style applied to their notes

**Is your feature request related to a problem? Please describe.**

The problem is that the default CSS style that sets the H1 underline cannot be removed; at least from my own knowledge.

**Describe the solution you'd like**
A button at the bottom of the editor that toggles whether or not style any of the MD file.

**Describe alternatives you've considered**

I've created the below document that does everything, I think*, that I want to achieve; an MD document with full control of the document's styles such as the border-bottom in the H1 and fonts. Plus the defaults automatically bold the H1, and may have other defaults I'd like to wipe clean.

```

h1 {border-bottom: none !important;}
#md-document {all:initial;}

Because the `all: initial;` doesn't reset everything, and things like the h1's border-bottom need to individually be overridden I'd like a simpler way to not apply HackMD's default styles.

**Additional context**
My solution doesn't override the automatic bolding, I'm not sure how I would do that while still being able to use `****` to apply bolding.

Inizia leggendo il CSS predefinito dell’editor e il modo in cui gli stili vengono applicati alle note Markdown. Definisci l’ambito e il comportamento dell’interruttore, quindi verifica che la sua disattivazione rimuova lo stile predefinito delle note, preservando al contempo la formattazione Markdown e gli stili forniti dall’utente.

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