Implement configurable update rate #30

Should be able to set how often the sources, widgets, and/or LCD updates

Ideas:

• Global update rate for the application. e.g. sources and screen are updated 10x a second
• Independent update rates for sources. Some may be heavier to check than others
• Independent update rates for widgets. Some things probably don't need to be updated as frequently. This could be set either directly or as a multiple of the global update rate