Add ITK_GIT_TAG and ITK_GIT_REPOSITORY variables for Superbuild
Add ITK_GIT_TAG and ITK_GIT_REPOSITORY variables for Superbuild

Adding these advanced options to the SimpleITK Superbuild allows for
easy experimentation against development versions of ITK and usage of
faster repository. The ITK_REPOSITORY variable is now a legacy option
in favor of ITK_GIT_REPOSITORY.

richardbeare/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,kaspermarstal/SimpleElastix,richardbeare/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,richardbeare/SimpleITK,blowekamp/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,kaspermarstal/SimpleElastix,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,kaspermarstal/SimpleElastix,SimpleITK/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,kaspermarstal/SimpleElastix