Allow specifying a system wide kitty config file

The system config is merged with any user specific config files, with
options in the user specific config files having higher priority.

Also follow the full XDG spec for finding config files, including
XDG_CONFIG_DIRS.

kitty/cli.py

@@ -11,7 +11,6 @@ from .config import load_config
 from .constants import appname, defconf, is_macos, str_version
 from .layout import all_layouts
 
-is_macos
 OPTIONS = '''
 --class
 dest=cls
@@ -32,9 +31,21 @@ only use this if you are running a program that does not set titles.
 
 --config
 type=list
-default={config_path}
 Specify a path to the configuration file(s) to use.
-Can be specified multiple times to read multiple configuration files in sequence, which are merged.
+Can be specified multiple times to read multiple configuration files in
+sequence, which are merged.  Use the special value NONE to not load a config
+file.
+
+If this option is not specified, config files are searched for in the order:
+"$XDG_CONFIG_HOME/kitty/kitty.conf", "~/.config/kitty/kitty.conf", {macos_confpath}
+"$XDG_CONFIG_DIRS/kitty/kitty.conf". The first one that exists is used as the
+config file.
+
+If the environment variable "KITTY_CONFIG_DIRECTORY" is specified, that
+directory is always used and the above searching does not happen.
+
+If "/etc/xdg/kitty/kitty.conf" exists it is used as a base config file onto
+which any user config files are merged.
 
 
 --override -o
@@ -201,10 +212,10 @@ def parse_option_spec(spec=OPTIONS):
                     current_cmd['choices'] = {x.strip() for x in current_cmd['choices'].split(',')}
         elif state is HELP:
             if line:
-                current_cmd['help'] += ' ' + line
+                current_cmd['help'] += ' ' + line.lstrip()
             else:
                 if prev_line:
-                    current_cmd['help'] += '\n'
+                    current_cmd['help'] += '\n\n\t'
                 else:
                     state = NORMAL
                     (seq if current_cmd.get('condition', True) else disabled).append(current_cmd)
@@ -453,7 +464,7 @@ def parse_cmdline(oc, disabled, args=None):
 def options_spec():
     if not hasattr(options_spec, 'ans'):
         options_spec.ans = OPTIONS.format(
-            appname=appname, config_path=defconf,
+            appname=appname, macos_confpath='~/Library/Preferences/kitty/kitty.conf' if is_macos else '',
             window_layout_choices=', '.join(all_layouts)
         )
     return options_spec.ans
@@ -466,8 +477,22 @@ def parse_args(args=None, ospec=options_spec, usage=None, message=None, appname=
     return parse_cmdline(oc, disabled, args=args)
 
 
+SYSTEM_CONF = '/etc/xdg/kitty/kitty.conf'
+
+
+def resolve_config(config_files_on_cmd_line):
+    if config_files_on_cmd_line:
+        if 'NONE' not in config_files_on_cmd_line:
+            yield SYSTEM_CONF
+            for cf in config_files_on_cmd_line:
+                yield cf
+    else:
+        yield SYSTEM_CONF
+        yield defconf
+
+
 def create_opts(args):
-    config = args.config or (defconf, )
+    config = resolve_config(args.config)
     overrides = (a.replace('=', ' ', 1) for a in args.override or ())
     opts = load_config(*config, overrides=overrides)
     return opts