Added search filter for task priority

app/Filter/TaskPriorityFilter.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Kanboard\Filter;
+
+use Kanboard\Core\Filter\FilterInterface;
+use Kanboard\Model\TaskModel;
+
+/**
+ * Class TaskPriorityFilter
+ *
+ * @package Kanboard\Filter
+ * @author  Frederic Guillot
+ */
+class TaskPriorityFilter extends BaseFilter implements FilterInterface
+{
+    /**
+     * Get search attribute
+     *
+     * @access public
+     * @return string[]
+     */
+    public function getAttributes()
+    {
+        return array('priority');
+    }
+
+    /**
+     * Apply filter
+     *
+     * @access public
+     * @return FilterInterface
+     */
+    public function apply()
+    {
+        $this->query->eq(TaskModel::TABLE.'.priority', $this->value);
+        return $this;
+    }
+}