Issue title: GraphQLRuleTester not available
 Issue body: **Describe the bug**

The GraphQLRuleTester class described in the custom rules docs (https://github.com/dotansimha/graphql-eslint/blob/master/docs/custom-rules.md#testing-your-rules) isn't available.

**To Reproduce**

Install `@graphql-eslint/eslint-plugin` locally, and run the following in a node repl. The object is undefined

```
> const { GraphQLRuleTester } = require("@graphql-eslint/eslint-plugin")
undefined
```

**Expected behavior**

I'd expect this to be exported so I can write a test as documented.

**Environment:**

- OS: macOS
- `@graphql-eslint/...`: `"@graphql-eslint/eslint-plugin": "^1.0.1",`
- NodeJS: 12.22.1


 Comments: 
 Comment 0: oops, definitely a missing export! 

Fixed in `@zachary21@example.com`. 

{'source': 'GithubIssues', 'repo_name_with_owner': 'dotansimha/graphql-eslint', 'issue_id': 925035117, 'repo_stars': 357.0}