Markdown editor headers buttons and customisation

Hi there,
I've added some missing buttons in the Markdown toolbar (H1 to H6).
Moreover, I've added an option to the MD field: `toolbarOptions`.
That option is an object that could accept several keys in order to customise the toolbar. Currently, I've implemented only the `hiddenButtons` key (it is a comma separated list of buttons to hide).
```
toolbarOptions: { hiddenButtons: 'H1,Image,Quote' }
```

I've also updated the docs with an example.