How to handle custom types?

How should I handle models with custom types such as [`mongoose-long`]?

Currently I get the following error when creating a `TypeComposer` with `composeWithMongoose` if the model has a field using the custom type `mongoose.Schema.Types.Long`.

```
Uncaught Exception: Error: You provide incorrect mongoose field to `deriveComplexType()`. Correct field should contain `path` and `instance` properties.
at deriveComplexType (/code/node_modules/graphql-compose-mongoose/lib/fieldsConverter.js:228:11)
at convertFieldToGraphQL (/code/node_modules/graphql-compose-mongoose/lib/fieldsConverter.js:197:23)
at Object.keys.forEach.fieldName (/code/node_modules/graphql-compose-mongoose/lib/fieldsConverter.js:145:13)
at Array.forEach ()
at convertModelToGraphQL (/code/node_modules/graphql-compose-mongoose/lib/fieldsConverter.js:142:31)
at composeWithMongoose (/code/node_modules/graphql-compose-mongoose/lib/composeWithMongoose.js:30:57)
```

[`mongoose-long`]: https://www.npmjs.com/package/mongoose-long

Start with fieldsConverter.js, especially deriveComplexType() and convertFieldToGraphQL(), then trace how composeWithMongoose() passes model fields into them. Reproduce the failure with a mongoose.Schema.Types.Long field and verify that custom types can be converted without the reported exception.

Written by the indexing model from the issue text.