Add generate.additional-tags config option

[cubahno]

Summary

Closes #77

Adds a new generate.additional-tags config option that emits additional struct tags on generated fields, mirroring the json tag value.

This solves the problem where YAML libraries like go.yaml.in/yaml/v4 silently fail to unmarshal multi-word properties (e.g. closed_at) because they lowercase the Go field name without preserving underscores (closedat instead of closed_at).

Usage

generate:
  additional-tags:
    - yaml

Generates:

type Issue struct {
    ClosedAt  time.Time `json:"closed_at" yaml:"closed_at"`
    LabelName *string   `json:"label_name,omitempty" yaml:"label_name,omitempty"`
}