Allow for querying a person by slug

[cdriehuys]

Anyone should be able to fetch a person's information by the person's slug.

query GetPersonBySlug($slug: String!) {
  person(slug: $slug) {
    name
  }
}

Acceptance Tests

1. Get By Slug

If there is a person with the provided slug, their information should be returned.

2. Invalid Slug

If there is no person with the provided slug, an error message should be returned.