Support setting categories on posts

category.go

@@ -0,0 +1,8 @@
+package writeas
+
+// Category represents a post tag with additional metadata, like a title and slug.
+type Category struct {
+	Hashtag string `json:"hashtag"`
+	Slug    string `json:"slug"`
+	Title   string `json:"title"`
+}

post.go

@@ -52,7 +52,8 @@ type (
 		IsRTL    *bool      `json:"rtl,omitempty"`
 		Language *string    `json:"lang,omitempty"`
 
-		AuthorSlug *string `json:"author"`
+		AuthorSlug *string    `json:"author,omitempty"`
+		Categories []Category `json:"categories,omitempty"`
 
 		// Parameters only for creating
 		Crosspost []map[string]string `json:"crosspost,omitempty"`