Single character literals do not work

[ErisDS]

Originally reported at TryGhost/GQL#24 & TryGhost/Ghost#8433

---

To reproduce:

• On a local blog...
• Create a post with the title A, the slug will be a. Publish the post.
• Create a post with the title AB, the slug will be ab. Publish the post.
• Visit the frontend of the blog to grab the client secret.
• Open postman
• Try doing a get request for http://localhost:2368/ghost/api/v0.1/posts?client_id=ghost-frontend&client_secret=[client-secret]&filter=slug:ab - see that it works
• Try doing a get request for http://localhost:2368/ghost/api/v0.1/posts?client_id=ghost-frontend&client_secret=[client-secret]&filter=slug:a - see that it doesn't work

I believe this is due to the plus sign here: https://github.com/TryGhost/GQL/blob/master/src/gql.l#L22

The badcharsincnot regex matches a single character, and then the second group matches a second character. Therefore there must be at least 2 chars for a match.

Fixing this may have other implications, if so, we should close this issue and instead, add a rule that literals must be at least 2 characters to the documentation, provide tests to demo this limitation + if possible, improve the error message.