Add: Exposure Module (#721)

* exposure module

* requested changes

* update module

src/Modules.js

@@ -18,6 +18,7 @@ module.exports = {
   'draw-rectangle': require('./modules/DrawRectangle'),
   'dynamic': require('./modules/Dynamic'),
   'edge-detect': require('./modules/EdgeDetect'),
+  'exposure': require('./modules/Exposure'),
   'flip-image': require('./modules/FlipImage'),
   'fisheye-gl': require('./modules/FisheyeGl'),
   'histogram': require('./modules/Histogram'),