Fix Func for the gondola gen cmd
Slight doc change. Mentioning chaining does not make sense in this func.
Reorder imports
Add shorthand syntax for including struct fields in json gen
Fix error parsing error responses from twitter
Add support for file inputs in gnd.la/form

Use an specific type, named gnd.la/form.File which can be embedded
in structs and is rendered as a file.
Remove the invalid field in Form

Instead, iterate over the fields and check if any of them has errors.
This fixes a bug which caused not chosen fields to invalidate the
form, while also making pretty much difficult to make this same
error in the future.