Add support for SUPERBUILD_ADDITIONAL_FOLDERS

This adds a new CMake cache variable SUPERBUILD_ADDITIONAL_FOLDERS that lets ParaView superbuild users specify one or more folders to look for more superbuild architecture.

The build option, SUPERBUILD_ADDITIONAL_FOLDERS is a semicolor separated list of folders to look for additional superbuild architecture, which enable ParaView superbuild user to build, install and package projects (including ParaView plugins) which are not part of the ParaView superbuild.

The main use cases it to be able to create a ParaView package using the ParaView superbuild with a custom ParaView plugin integrated into the package without requiring manual modification of the generated archives.

See README.md in the diff for complete info.