Add Gist code search (#194)

internal/index/gist.go

@@ -0,0 +1,21 @@
+package index
+
+type Gist struct {
+	GistID     uint
+	Username   string
+	Title      string
+	Content    string
+	Filenames  []string
+	Extensions []string
+	Languages  []string
+	CreatedAt  int64
+	UpdatedAt  int64
+}
+
+type SearchGistMetadata struct {
+	Username  string
+	Title     string
+	Filename  string
+	Extension string
+	Language  string
+}