This patch adds the new APRSD Command Plugin architecture.
All Comand plugins must implement the same object API, which includes
plugin object is subclass of APRSDPluginBase
version attribute
command_regex attribute
command method
When an APRS command is detected, then the regex is run against
the command. If the command_regex matches, then the plugin's
command() method will be called. If the command() method returns
a string, then that string is sent as a reply to the APRS caller.
A new aprs.yml config section is added to support selecting
which plugins to enable.
If you want all plugins enabled, then omit "enabled_plugins" entirely
from the aprs section of the config.
To load custom plugins:
1) create a directory with an __init__.py file
2) Add a plugin.py file that contains your plugin
Look at the exmaples directory for an example plugin.
