Add useBaseInternationalization to SpecOptions (#961)

* Add 'useBaseInternationalization' setting to SpecOptions (default value of true)

* Update PBXProjGenerator to only include Base into knownRegions if it was either detected on the filesystem or if the project spec options opt into it

* Update ProjectSpec.md to include useBaseInternationalization

* Update AnotherProject to demonstrate Base Internationalization opt out

* Update CHANGELOG.md

Sources/XcodeGenKit/PBXProjGenerator.swift

@@ -292,7 +292,12 @@ public class PBXProjGenerator {
             projectAttributes["knownAssetTags"] = assetTags
         }
 
-        pbxProject.knownRegions = sourceGenerator.knownRegions.union(["Base", developmentRegion]).sorted()
+        var knownRegions = Set(sourceGenerator.knownRegions)
+        knownRegions.insert(developmentRegion)
+        if project.options.useBaseInternationalization {
+            knownRegions.insert("Base")
+        }
+        pbxProject.knownRegions = knownRegions.sorted()
 
         pbxProject.packages = packageReferences.sorted { $0.key < $1.key }.map { $1 }