Poll for config changes

[robozman]

We need to implement a way for sources to be periodicly checked for changes. Maybe we can use filesystem timestamps for local configs? Potential solution for remote (or both for simplicity / code reuse) is to calculate a hash (how?) for the source on update and compare it to version pointed to by the config.

We also need to decide how we'll poll visplay.yaml for changes and see if we want to roll it into this process.

Python / code wise, I think we might have to look into threading for this.