Adds plugin for Contentful synchronization (#75)

Adds plugin for Contentful

Will create product and product variants in Contentful, when created in Medusa

Closes #47

packages/medusa-plugin-contentful/src/subscribers/contentful.js

@@ -0,0 +1,20 @@
+class ContentfulSubscriber {
+  constructor({ contentfulService, eventBusService }) {
+    this.contentfulService_ = contentfulService
+    this.eventBus_ = eventBusService
+
+    this.eventBus_.subscribe("product-variant.updated", async (data) => {
+      await this.contentfulService_.updateProductVariantInContentful(data)
+    })
+
+    this.eventBus_.subscribe("product.updated", async (data) => {
+      await this.contentfulService_.updateProductInContentful(data)
+    })
+
+    this.eventBus_.subscribe("product.created", async (data) => {
+      await this.contentfulService_.createProductVariantInContentful(data)
+    })
+  }
+}
+
+export default ContentfulSubscriber