allow splittable user to use additional plugins in the deps finding pipeline

this is needed as there are some constructs that the jison library
generates that blows up with "use strict" directive. I need to be able
to pass "transform-remove-strict-mode" plugin

Author: erwinmombay

splittable.js

@@ -238,9 +238,10 @@ exports.getGraph = function(entryModules, config) {
   // directly and which we don't want to apply during deps finding.
   transform(babel, {
     babelrc: false,
-    plugins: [
+    plugins: Array.isArray(config.babel.plugins) ? config.babel.plugins.concat([
       require.resolve("babel-plugin-transform-es2015-modules-commonjs"),
-    ]
+    ]) : [require.resolve("babel-plugin-transform-es2015-modules-commonjs"),
+]
   });
 
   b.on('package', function(pkg) {