Add scrollable property to modal

[DaleCam]

Adds optional property scrollable to modal. Fixes issue #61
Important!:
I wasn't able to figure out how to build/serve this to visually test it. Feedback on how to build so i can test locally would be appreciated, however the code is covered by new unit tests I added and they are working as expected.

[Morgul]

The implementation looks reasonable. Regarding building... yeah, I'm not even sure I can currently build the project; I've gone through two machines since I last tried.

This weekend, I'll put some effort into getting it to build and see if I can't get a release out with these changes.

[Drumstix42]

Changes look good. Would love to see this make it into the code

As an aside:
It might be good to [also] allow the flexibility of custom class names to make it onto the same element as .modal-dialog. I'm all for easy to use property booleans, but there are some other helpers, and sometimes you may run into a situation where you'd like to supplement these with an additional class name. For a basic example though, Bootstrap 4 documents notes you can do things such as:

<!-- Vertically centered scrollable modal -->
<div class="modal-dialog modal-dialog-centered modal-dialog-scrollable">
  ...
</div>

Edit
For anyone reaching this comment, another user pointed out you can "hijack" the "size" value and put in other class names, as it's just string that gets placed in the appropriate element.
#58 (comment)

[Drumstix42]

@Morgul Will you be making a new release/tag now that this is merged and completes the related Milestone?

[Morgul]

New version should be on npm. Let me know if there's issues, it's been two years since I've done this with this project, and nothing about it's standard. 😐