Let users configure custom file type mappings

[sternenseemann]

We have a map like this:

M.Map FilePath GopherFileType

The configuration option could look like this, allowing to override the mapping:

{
  "file-types": {
    "apng": "I"
  }
}

Also the default fallback should be configurable (currently File).

[hyperrealgopher]

https://github.com/hyperrealgopher/filesig

Just dropping this here in case it's useful.