Expose vips internal cache settings and status

tests/parallel.js

@@ -26,4 +26,6 @@ async.mapSeries([1, 1, 2, 4, 8, 16, 32, 64, 128], function(parallelism, next) {
       next();
     }
   );
-}, function() {});
+}, function() {
+  console.dir(sharp.cache());
+});

tests/perf.js

@@ -420,4 +420,5 @@ async.series({
   Object.keys(results).forEach(function(format) {
     assert.strictEqual("sharp", results[format].toString().substr(0, 5), "sharp was slower than " + results[format] + " for " + format);
   });
+  console.dir(sharp.cache());
 });