Implementing plugins for demo
- Add selected plugin source files to git
- Support "fire = yes/no" option
- Label log messages more clearly
- Add '' as a key in map_time_unit so seconds is the default
Described in previous commit message
 - Unignore plugins directory
Configuration defaults
- Add default values to the config object constructed by get_config()
- Add new options to the sample config file ('heartbeat', 'fire', one
  representing arbitrary options in plugin sections)
- Document get_config()
Detect config changes and reload.
 - Beat the heart on the modulo of the heartbeat duration
 - Update cfgdump tests to fail if default values are reported