Reorganize hooks into dedicated submodule

Fixes #93

tox_poetry_installer/hooks/tox_add_option.py

@@ -0,0 +1,16 @@
+"""Add additional command line arguments to tox to configure plugin behavior"""
+from tox.config.cli.parser import ToxParser
+from tox.plugin import impl
+
+from tox_poetry_installer import constants
+
+
+@impl
+def tox_add_option(parser: ToxParser):
+    parser.add_argument(
+        "--parallel-install-threads",
+        type=int,
+        dest="parallel_install_threads",
+        default=constants.DEFAULT_INSTALL_THREADS,
+        help="Number of locked dependencies to install simultaneously; set to 0 to disable parallel installation",
+    )