Created plugin.py for Command Plugins

[hemna]

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.

[hemna]

FWIW, the command_email hasn't been migrated to a core plugin, because it calls send_message() in a few places internally. Need to eventually refactor the messaging code into it's own file and email functions into it's own file. If the email command needs to call send_message() then it can simply call return message in the function. I just haven't looked at the function that much to determine if it's done after it calls send_message().

So for now command_email stays as a COMMAND_ENVELOPE mechanism.