Migrate to Material UI v4

[Verbon]

Hi,

Material UI v4 has come out a few months ago (https://medium.com/material-ui/material-ui-v4-is-out-4b7587d1e701).
So, please migrate mui-downshift to support Material UI v4.
Currently the following warning is displayed in console when installing a package:

warning "... > mui-downshift@1.4.1" has incorrect peer dependency "@material-ui/core@^3.0.0".
warning "... > mui-downshift@1.4.1" has incorrect peer dependency "@material-ui/icons@^3.0.0".

Here you can find the migration guide - https://material-ui.com/guides/migration-v3.

[techniq]

See: #80 and #83.

I currently don't have the time to do the work myself, and await a PR that does not have errors / regressions. #83 was getting close but still had some errors. Most of the discussion was taking place in #80 but the best PR is #83.