Support gifs in ideal image plugin

## 🐛 Bug Report

Plugin itsel supports .gif (on webpack configuration) but this plugin doesn't work with gif https://github.com/endiliey/responsive-loader (i guess)

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

Yes

## To Reproduce

```jsx

```

## Expected behavior
To work correctly.

## Actual Behavior

```sh
./static/img/real-code.gif
Error: No mime type for file with extension gifsupported
@ ./src/pages/index.tsx 1:9091-9132
@ ./.docusaurus/registry.js
@ ./node_modules/@docusaurus/core/lib/client/exports/ComponentCreator.js
@ ./.docusaurus/routes.js
@ ./node_modules/@docusaurus/core/lib/client/clientEntry.js
@ multi ./node_modules/react-dev-utils/webpackHotDevClient.js ./node_modules/@docusaurus/core/lib/client/clientEntry.js
```

## Your Environment

- Docusaurus version used: alpha@70