Add ability to lookup via CLI.

    python -m intersphinx_registry.lookup [package names]

Not direction -m intersphinx_registry in case we want to add
functionalities later.

intersphinx_registry/lookup.py

@@ -0,0 +1,20 @@
+import sys
+
+from intersphinx_registry import get_intersphinx_mapping
+from sphinx.ext.intersphinx import inspect_main
+
+if len(sys.argv) < 2:
+    sys.exit("Usage: python -m intersphinx_registry.lookup [packages...]")
+
+packages = sys.argv[1:]
+
+urls = [
+    u[0] + (u[1] if u[1] else "objects.inv")
+    for u in get_intersphinx_mapping(packages=packages).values()
+]
+
+
+import logging as _logging
+
+_logging.basicConfig()
+raise SystemExit(inspect_main(urls))