initially added Content interface and TextContent child interface

src/main/kotlin/com/github/insanusmokrassar/postssystem/core/content/Content.kt

@@ -0,0 +1,9 @@
+package com.github.insanusmokrassar.postssystem.core.content
+
+import org.joda.time.DateTime
+
+typealias ContentId = String
+
+interface Content {
+    val id: ContentId
+}

src/main/kotlin/com/github/insanusmokrassar/postssystem/core/content/TextContent.kt

@@ -0,0 +1,5 @@
+package com.github.insanusmokrassar.postssystem.core.content
+
+interface TextContent : Content {
+    val text: String
+}