Skip creating encoders for unexported fields. They are not participating
in JSON marshaling anyway. This allows using unexported fields of
non-marshalable types in structs.
As a side-effect of this change it's no longer possible to marshal
unexported JSON fields by adding a custom type extenstion. It seems this
is desired behavior since it matches standard library and jsoniter
already disallows `json:"-"` fields from participating in custom
extensions.
Fixes#174.
