Allow custom keys for slider monitor

[okorz001]

This is mostly lifted from redux-devtools-dock-monitor, but there was some extra complexity around the fact that the underlying parse-key library doesn't support ctrl-[ and ctrl-]. Ideally, I would have defaulted the new props to the default bindings, but I had to settle for falling back to original logic when they are unset.

I couldn't get the example todomvc project running, but I was able to verify these changes in my own project, both with and without the new props.

Closes #471.

[okorz001]

The build failure doesn't have anything to do with my changes. There's some node-gyp failures, probably related to node 12 being released.