Make it possible to run custom kittens via kitty +kitten

Should help reduce confusion about how kittens work. Fixes #1712

kittens/runner.py

@@ -6,6 +6,7 @@
 import importlib
 import os
 import sys
+from contextlib import suppress
 from functools import partial
 
 aliases = {'url_hints': 'hints'}
@@ -15,13 +16,18 @@ def resolved_kitten(k):
     return aliases.get(k, k).replace('-', '_')
 
 
-def import_kitten_main_module(config_dir, kitten):
-    if kitten.endswith('.py'):
-        path_modified = False
+def path_to_custom_kitten(config_dir, kitten):
     path = os.path.expanduser(kitten)
     if not os.path.isabs(path):
         path = os.path.join(config_dir, path)
     path = os.path.abspath(path)
+    return path
+
+
+def import_kitten_main_module(config_dir, kitten):
+    if kitten.endswith('.py'):
+        path_modified = False
+        path = path_to_custom_kitten(config_dir, kitten)
         if os.path.dirname(path):
             sys.path.insert(0, os.path.dirname(path))
             path_modified = True
@@ -90,16 +96,24 @@ def deserialize(output):
 
 def run_kitten(kitten, run_name='__main__'):
     import runpy
+    original_kitten_name = kitten
     kitten = resolved_kitten(kitten)
     set_debug(kitten)
-    try:
+    with suppress(ImportError):
         runpy.run_module('kittens.{}.main'.format(kitten), run_name=run_name)
-    except ImportError:
-        raise SystemExit((
-            'No kitten named {}. If you are trying to run a custom kitten,'
-            ' you have to do so via a mapping in kitty.conf, see'
-            ' https://sw.kovidgoyal.net/kitty/kittens/custom.html'
-        ).format(kitten))
+        return
+    # Look for a custom kitten
+    if not kitten.endswith('.py'):
+        kitten += '.py'
+    from kitty.constants import config_dir
+    path = path_to_custom_kitten(config_dir, kitten)
+    if not os.path.exists(path):
+        print('Available builtin kittens:', file=sys.stderr)
+        for kitten in all_kitten_names():
+            print(kitten, file=sys.stderr)
+        raise SystemExit('No kitten named {}'.format(original_kitten_name))
+    m = runpy.run_path(path, init_globals={'sys': sys, 'os': os}, run_name='__run_kitten__')
+    m['main'](sys.argv)
 
 
 def all_kitten_names():