Support font awesome with fas and far class

Hi, I'm a big fan of hackmd.io!!

When I looked in to the [feature document](https://hackmd.io/s/features), I noticed that you can use *font awesome* by inserting a simple line of html code. like this: ``

However, this only works when the class is `fa`, when it comes to `fas` or `far` class then it shouldn't work.

Is it possible to support those classes? (Or how can I help? I only have some basics about html.)

I'm wondering if this thread is related? https://github.com/hackmdio/codimd/issues/872

Thanks for any suggestion!