Fixed issue with templates

Initial template strategy used a fixed order of rendering. When adding custom template entries, that order could not be controlled, making it hard/impossible to customize templates without breaking things.

• Render templates as needed
• Improved template documentation
• Added templating example

Noteworthy changes

• location_tracker and location are no longer required. If omitted, the Home location of Home Assistant will be used
• flights_entity is no longer required. Default value is set to sensor.flightradar24_current_in_area
• no_flights_message can be configured. Set to empty string '' to disable message.

Full Changelog: v0.0.6...v0.0.7