Fix tests, add description how to start dynamic into readme

Author: randomhash

jest-mongodb-config.js

@@ -3,6 +3,7 @@ module.exports = {
     binary: {
       skipMD5: true
     },
-    autoStart: false
+    autoStart: false,
+    instance: {}
   }
 };

readme.md

@@ -34,6 +34,7 @@ module.exports = {
       skipMD5: true
     },
     autoStart: false
+    instance: {}
   }
 };
 ```
@@ -50,6 +51,27 @@ module.exports = {
       version: '4.0.3',
       skipMD5: true
     },
+    instance: {
+      dbName: 'jest'
+    }
+    autoStart: false
+  }
+};
+```
+
+To use dynamic database name you must pass empty object for instance field
+
+```js
+module.exports = {
+  mongodbMemoryServerOptions: {
+    instance: {
+      dbName: 'jest'
+    },
+    binary: {
+      version: '4.0.3',
+      skipMD5: true
+    },
+    instance: {}
     autoStart: false
   }
 };

setup.js

@@ -39,7 +39,8 @@ function getMongodbMemoryOptions() {
       binary: {
         skipMD5: true
       },
-      autoStart: false
+      autoStart: false,
+      instance: {}
     };
   }
 }