Player enhancements - Ability to have the player docked to header or footer location on site.

As a followup to enhancements done in https://github.com/10up/simple-podcasting/pull/272

It might be nice to enhance the player with the ability to have the player docked to header or footer location on site (Ref: [prior art](https://10up.com/blog/2016/wamu-react-rest-api/) on [WAMU's site](https://wamu.org/) and bottom docked player experience).