Add "Conditional Executution" plugin.

Change-Id: I4fd30159ea93503c7354d07739ffdcb6a1dedebb
Reviewed-on: http://gerrit.dmdirc.com/2629
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>
Reviewed-by: Greg Holmes <greg@dmdirc.com>