Is it possible to add contributors who don't have a GitHub account?

**Is your feature request related to a problem? Please describe.**
I appreciate this is an unusual request, but I have been asked if it is possible to add people with a GitHub account.

> How do you add people without a GitHub handle?

**Describe the solution you'd like**
A method to add people to @all-contributors without a GitHub username. I guess the syntax would be along the lines of

```
@all-contributors please add "Test User" for code
```

**Describe alternatives you've considered**
Could manually adjust the README table, or just write in plain text somewhere. Or ask users to create a GitHub account.