Issue title: Can you use without advanced configuration?
 Issue body: Hi!

I was trying to use this plugin but without the jss insertion specified here https://www.gatsbyjs.org/packages/gatsby-plugin-material-ui/#advanced you can't override styles properly. I think this should be done by default instead of requiring every project to setup another extra js file of config.
 Comments: 
 Comment 0: What's your use case? 
 Comment 1: Just override a style with emotion.js. You can't use this plugin out of the box, you need to use advanced mode. I think that should be by default as most of the people need to override material ui styles.
 Comment 2: Ok, thanks. I have added your comment in https://trello.com/c/QZN9Zz69/2557-https-githubcom-mui-org-material-ui-issues-20776issuecomment-619633328. We might change the default injection order in v5, we will see.
 Comment 3: This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

 Comment 4: This issue has been automatically closed because of inactivity. Please open a new issue if are still encountering problems.

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