Update submodules: plugins (new commits)

plugins:
    a21a51b Greg Holmes <greg@dmdirc.com>
            Don't set foreground of null objects.

            I should probably fix this is a nicer way, but this works.

            Gerrit: http://gerrit.dmdirc.com/3758

Change-Id: I14166f19430d364c42f53ef26bd4015c8af4bb37