Always skip unexported fields when encoding

[olegshaldybin]

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.

[olegshaldybin]

@taowen I see there are some tests that use custom extension to marshal/unmarshal private fields... not sure this is a good pattern but if you want to keep that behavior then we need a slightly more advanced fix for #174, maybe adding an ignored flag for the fields of unsupported types in the struct descriptor.

[taowen]

if field is unexported, the field names will be empty slice. field names are used to decode or encode one go field from multiple json field. if is empty, meaning it is not present in json.

[ghost]

Feature request: we can custom define the unsupport value field,
json.setUnsupportFieldValue("null");

or
json.setUnspportFieldValue("")