text: Title: Make it possible to specify the plain text email template
  Body: # Prerequisites
  
  * [ ] Is it a bug?
  * [x] Is it a new feature?
  * [ ] Is it a a question?
  * [ ] Can you reproduce the problem?
  * [x] Are you running the latest version?
  * [x] Did you check for similar issues?
  * [x] Did you perform a cursory search?
  
  For more information, see the [CONTRIBUTING](https://github.com/PedroBern/django-graphql-auth/blob/master/CONTRIBUTING.md) guide.
  
  # Description
  
  Right now the plain text version of the email is generated on the fly, it would be nice to be able to use a plain text template.