Add ability to filter posts by slug with API

- This takes care of some of the discussion in the following commit:
    - https://github.com/Codrspace/codrspace/commit/138708b2fb9af0b19f89ebef0d8f280c37e4e519
    - SHA: 138708b2fb9af0b19f89ebef0d8f280c37e4e519