Added the ability to "toggle" apps

Added the enable and disable commands. When supplied with a list of apps, the enable and disable commands edits the configuration file for mackup. If using the enable command, then it adds the app names to the "applications_to_sync" section. If using the disable command, then it adds the app names to the "applications_to_ignore" section. Before writing to the configuration file, the app names are removed from BOTH sections so that neither conflicts or duplicates are written to the configuration file.