Added remote command: FROM_FILE

Remote commands can be read directly from a file (one per line).
Lines starting with `#` and empty lines are ignored.

test/test_parsing_files_with_commands.py

@@ -0,0 +1,47 @@
+from test.picardtestcase import PicardTestCase
+
+from picard.tagger import Tagger
+
+
+class TestParsingFilesWithCommands(PicardTestCase):
+
+    MOCK_FILE_CONTENTS = (
+        # should be split into 2 commands
+        "LOAD file1.mp3 file2.mp3",
+        # should be added as one
+        "FROM_FILE file0.mp3",
+        "CLUSTER",
+        "  FINGERPRINT  "
+        # should be ignored
+        "",
+        " ",
+        "\n",
+        "#commented command",
+    )
+
+    def setUp(self):
+        super().setUp()
+        self.result = tuple(x for x in Tagger._parse_commands_from_lines(self.MOCK_FILE_CONTENTS))
+
+    def test_no_argument_command(self):
+        self.assertIn("command://CLUSTER ", self.result)
+
+    def test_no_argument_command_stripped_correctly(self):
+        self.assertIn("command://FINGERPRINT ", self.result)
+
+    def test_single_argument_command(self):
+        self.assertIn("command://FROM_FILE file0.mp3", self.result)
+
+    def test_multiple_arguments_command(self):
+        self.assertIn("command://LOAD file1.mp3", self.result)
+        self.assertIn("command://LOAD file2.mp3", self.result)
+
+    def test_empty_lines(self):
+        self.assertNotIn("command:// ", self.result)
+        self.assertNotIn("command://", self.result)
+        # 1 FROM_FILE
+        # 2 LOADs
+        self.assertEqual(len(self.result), 5)
+
+    def test_commented_lines(self):
+        self.assertNotIn("command://#commented command", self.result)