CB-6469 - Restore plugins from config.xml

Adds a new save command to CLI which persists the currently added plugins to config.xml. There
is also an accompanying restore command which scans the config.xml and
restores the missing plugins that are listed.  Adds a new function to ConfigParser for adding
features. Updates plugin rm command to print a warning to also remove a
plugin that has been added to config.xml as a restore target.

Adds two new test for the new commands namely restore.spec and save.spec and
enhances the existing ConfigParser.spec for the new addFeature function.