[4chan] Add Support for 4chan (#142)

This commit adds support for 4chan. This means that 4chan can be
selected as a new source type. When the 4chan source is selected a user
can select a board from which he wants to get the RSS feed.

app/lib/widgets/source/add/add_source.dart

@@ -1,3 +1,4 @@
+import 'package:feeddeck/widgets/source/add/add_source_fourchan.dart';
 import 'package:flutter/material.dart';
 
 import 'package:feeddeck/models/column.dart';
@@ -45,6 +46,10 @@ class _AddSourceState extends State<AddSource> {
   /// user selected a source type, the functions returns the form for the
   /// selected source type.
   Widget _buildBody() {
+    if (_sourceType == FDSourceType.fourchan) {
+      return AddSourceFourChan(column: widget.column);
+    }
+
     if (_sourceType == FDSourceType.github) {
       return AddSourceGitHub(column: widget.column);
     }