`.HTMLConverter`: convert also placeholder and aria-label attributes.

[firmart]

The .HTMLConverter API works perfectly for text nodes, but it would be even nicer if it could also convert attributes like placeholder, aria-label etc.

Thanks.

[sgalal]

Thanks! Could you make a pull request?

[firmart]

I just had a look at the source code, it does seem fairly easy to achieve. Will look if I got time before next year... Thanks for the quick reply.