Custom style
What
There is an open issue to create enable users to customize the style of controls and annotations. This PR is covering an abstraction layer through OpenLayers so that users can use Microscopy Styling without touching OpenLayers directly. https://github.com/dcmjs-org/dicom-microscopy-viewer/blob/custom-style/src/style.js
This PR is not covering the feature to allow users to change some of the styles via CSS, for example, color or line thickness of selected or modified ROIs. I would recommend open a new PR latter only for this part.

There is a live example at https://custom-style--dicom-microscopy-viewer.netlify.com/styling/index.html
Why
We want to allow an application to customize styling, such that the style of library elements can be matched to that of the application.
LGTM 👍