add `getFormParam` for convenience
there is a `getPostParam` which is nice, 
but when uploading a file with a post, and the form enctype is changed to ` "multipart/form-data"`
one must use this long inconvenient code `ctx.request.formParams.data[key].body`, to access the formerly post params.

For such a case maybe consider adding a:

```nim
template getFormParam(ctx: Context, key: string): string =
  ctx.request.formParams.data[key].body
```