feat(client): expose feed and entry language on public Go client structs

Mirrors the new `language` JSON field on the server-side Feed and Entry
models so that third-party consumers of the Go API client can read the
value declared by feeds and entries.

client/model.go

@@ -147,6 +147,7 @@ type Feed struct {
 	SiteURL                     string    `json:"site_url"`
 	Title                       string    `json:"title"`
 	Description                 string    `json:"description"`
+	Language                    string    `json:"language"`
 	CheckedAt                   time.Time `json:"checked_at"`
 	NextCheckAt                 time.Time `json:"next_check_at"`
 	EtagHeader                  string    `json:"etag_header,omitempty"`
@@ -270,6 +271,7 @@ type Entry struct {
 	Title       string     `json:"title"`
 	Status      string     `json:"status"`
 	Content     string     `json:"content"`
+	Language    string     `json:"language"`
 	Author      string     `json:"author"`
 	ShareCode   string     `json:"share_code"`
 	Enclosures  Enclosures `json:"enclosures,omitempty"`