Add category pkg

This supports categories in Write.as / WriteFreely.

Ref T809

category/category.go

@@ -0,0 +1,30 @@
+// Package category supports post categories
+package category
+
+import (
+	"errors"
+	"github.com/writeas/slug"
+)
+
+var (
+	ErrNotFound = errors.New("category doesn't exist")
+)
+
+// Category represents a post tag with additional metadata, like a title and slug.
+type Category struct {
+	ID      int64  `json:"-"`
+	Hashtag string `json:"hashtag"`
+	Slug    string `json:"slug"`
+	Title   string `json:"title"`
+}
+
+// NewCategory creates a Category you can insert into the database, based on a hashtag. It automatically breaks up the
+// hashtag by words, based on capitalization, for both the title and a URL-friendly slug.
+func NewCategory(hashtag string) *Category {
+	title := titleFromHashtag(hashtag)
+	return &Category{
+		Hashtag: hashtag,
+		Slug:    slug.Make(title),
+		Title:   title,
+	}
+}