Adds optional list attributes

[nicofranzp]

Would close #58

[cmaas]

Thanks for taking the time to create a pull request. A bunch of things:

1. No tests.
2. If options.listAttrs is empty, the HTML output would be <ul > for every element (unnecessary whitespace).
3. tocItemToHtml is called recursively. Does it make sense that the same list of attributes is applied to sublists as well? I have no opinion on this, but we should be clear about it.
4. Would a general purpose transformListElement() function possibly solve the job in a better way?

I also had in mind to create a function that lets users rewrite the container class. If someone wants to use <nav> instead of <div> or something like <details><summary>..., the plugin would be a lot more flexible. But then it makes sense to have all customization in a similar format (custom formatting functions?).

What do you think?

[nicofranzp]

Hi, thanks for the reply!

1. I don't know how to provide a test 😬. Sorry! (literally my first PR)
2. I see the unnecessary whitespace...
3. I also understand why it might not be desirable to pass the same attributes to sublists and
4. yes, a transformListElemet() would have solved all my problems if it allows me to add specific classes to the first ul.

As my Wishlist and experience is no too big, I would add a couple of if statements to get rid of the white space and avoid adding the attributes into sublists... I know is not the most general way of doing things, but it is within my reach. I don't know if this would cause trouble ahed either.

Maybe a discussion of a what to expect from and how to implement a custom formatting functions might inspire people to jump in?