Use a template variable to support module @import syntax

Without this fix, I get a warning of "treating #import as an import of module 'CoreData' [-Wauto-import]". This change allows you to include the template variable as such: "--template-var modules=true" when running mogenerator to output the imports of CoreData in the new module syntax: @import CoreData;
Merge pull request #194 from danielctull-forks/import

[NEW] Specify `--template-var modules=true` option to avoid `treating #import as an import of module 'CoreData' [-Wauto-import]" warning`. (Daniel Tull)