Add ability to load filters from partials

Issue #12

src/Service/Partials.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace Opdavies\GmailFilterBuilder\Service;
+
+/**
+ * Load and combine partials from multiple files.
+ */
+class Partials
+{
+    /**
+     * Load partials from a directory.
+     *
+     * @param string $directoryName The name of the directory containing the partials.
+     *
+     * @return array The loaded filters.
+     */
+    public static function load($directoryName = 'filters')
+    {
+        $files = (new static())->getFilePattern($directoryName);
+
+        return collect(glob($files))
+            ->map(function (string $filename) {
+                return include $filename;
+            })
+            ->flatten(1)
+            ->all();
+    }
+
+    /**
+     * Build the glob pattern to load partials from the directory.
+     *
+     * Assumes it is always relative to the current directory, and that the
+     * filters are always in files with a .php extension.
+     *
+     * @param string $directoryName The name of the directory.
+     *
+     * @return string The full path.
+     */
+    protected function getFilePattern(string $directoryName)
+    {
+        return __DIR__ . DIRECTORY_SEPARATOR . $directoryName . DIRECTORY_SEPARATOR . '/*.php';
+    }
+}