Add ability for plugins to have their own private settings storage

Updates ReadSettingFromFile(), WriteSettingToFile(),
IfSettingEqualPrint(), and PrintSettingCheckbox() to take an
optional argument for the plugin name.  If this is set, we use
the settings file /home/pi/media/config/plugin.PLUGINNAME instead
of /home/pi/media/settings.  This keeps plugin settings separate
from the main FPPD settings and allows us to clean them up later
or 'reset' a plugin by removing its config file.