Convert or add anywidget implementation(?)

[cmdcolin]

The anywidget platform is created by member of gosling @manzt https://blog.jupyter.org/anywidget-jupyter-widgets-made-easy-164eb2eae102

Would be interesting if jbrowse-jupyter could use...I think I mentioned it to @garrettjstevens and it could even bypass some of the limitations of the dash widget system regarding the iframe maybe...