add --config flag to modify config path

+ use getters instead of constants in settings classes
+ refactor previous references
+ move get_xdg_config_dir() from settings.py to app.utils.xdg_utils.py

app/utils/xdg_utils.py

@@ -0,0 +1,21 @@
+import os
+
+
+def get_xdg_config_dir():
+    """
+    Returns the XDG_CONFIG_HOME environment variable if it exists, otherwise
+    returns the default config directory. If none of those exist, returns the
+    user's home directory.
+    """
+    xdg_config_home = os.environ.get("XDG_CONFIG_HOME")
+
+    if xdg_config_home:
+        return xdg_config_home
+
+    try:
+        alt_dir = os.path.join(os.environ.get("HOME"), ".config")
+
+        if os.path.exists(alt_dir):
+            return alt_dir
+    except TypeError:
+        return os.path.expanduser("~")