visidata: add IOC types with cached, throttled lookups

Centralize provider caching and rate-limit handling, then add Domain/URL/Hash IOC types and safer VT/IPInfo key resolution so lookups stay reliable on free-tier APIs.

config/visidata/plugins/__init__.py

@@ -10,10 +10,19 @@ their commands/types are registered on startup.
 for _mod in (
     "hidecol",
     "iptype",
+    "ioc",
 ):
     try:
         __import__(f"{__name__}.{_mod}")
-    except Exception:
-        # VisiData will show exceptions in its error sheet if needed; don't hard-fail here.
-        pass
+    except ModuleNotFoundError:
+        # Optional/missing plugin file.
+        continue
+    except Exception as e:
+        # Don't silently swallow unexpected import errors; surface them.
+        try:
+            from visidata import vd
 
+            vd.warning(f"plugin import failed: plugins.{_mod}")
+            vd.exceptionCaught(e)
+        except Exception:
+            raise