Add a build config for bundled plugins.

This is unused atm, but will be how the build script pulls plugins
into the jar.

Change-Id: Id777c8040a131a1171f03c7abc404cd6e03591f7
Reviewed-on: http://gerrit.dmdirc.com/4072
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>