Allow custom classes in editor

[n-studio]

The tag now accepts a hash of classes to customize each component.
Each component also has a new class ms--<name-of-the-component> for easy styling.

<%= marksmith_tag :body,
  disabled: true,
  placeholder: "Write your best markdown here.",
  extra_preview_params: { foo: "bar" },
  enable_file_uploads: true,
  upload_url: nil,
  classes: {
    textarea: "bg-red-500",
    editor_pane: "bg-blue-500",
    preview_pane: "bg-green-500",
    action_bar: "bg-yellow-500",
  }
  %>

The classes attribute is still retrocompatible if provided a String instead of a Hash, so this shouldn't be a breaking change.

For technical limitations, the rendered body cannot receive custom classes. Styling must be done through ms--rendered-body.

[adrianthedev]

I just merged #74.

I think this is a CSS issue and could be fixed with CSS and the semantic classes should help with that.

Would that PR fix the issues you're encountering?