Add settings repository interface and database implementation.

Almost done with flarum/core#121 now.

src/Core/DatabaseSettingsRepository.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Flarum\Core;
+
+use Illuminate\Database\ConnectionInterface;
+
+class DatabaseSettingsRepository implements SettingsRepositoryInterface
+{
+    protected $database;
+
+    public function __construct(ConnectionInterface $connection)
+    {
+        $this->database = $connection;
+    }
+
+    public function get($key, $default = null)
+    {
+        if (is_null($value = $this->database->table('config')->where('key', $key)->pluck('value'))) {
+            return $default;
+        }
+
+        return $value;
+    }
+
+    public function set($key, $value)
+    {
+        $this->database->table('config')->where('key', $key)->update(['value' => $value]);
+    }
+}

src/Core/SettingsRepositoryInterface.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Flarum\Core;
+
+interface SettingsRepositoryInterface
+{
+    public function get($key, $default = null);
+
+    public function set($key, $value);
+}