Change to using typed docstrings.

@ivarne there's the `:file` keyword in the metadata section that would be suitable for this I would think. 

```julia
@doc [ :file => "external_file.ext" ] ->
f(x) = x
```

Something like `writemime` but for reading from external sources, perhaps `readmime`, as a general mechanism to load file contents as the correct Julia type would be nice. I seem to remember an issue related to this in the Julia repo, but can't seem to locate it now.