move override implementation into own module

tests/test_override.py

@@ -0,0 +1,27 @@
+import pytest
+
+import mock
+
+from jrnl.args import parse_args
+from jrnl.jrnl import run, search_mode
+from jrnl import install
+from jrnl.override import apply_overrides
+@pytest.fixture()
+def minimal_config(): 
+    cfg = { 
+        "colors":{ 
+            "body":"red",
+            "date":"green"
+        },
+        "default":"/tmp/journal.jrnl",
+        "editor":"vim"
+    }
+    yield cfg 
+
+def test_apply_override(minimal_config): 
+    config = minimal_config.copy()
+    overrides = {
+        'editor':'nano'
+    }
+    config = apply_overrides(overrides, config)
+    assert config['editor']=='nano'