Make TOML optional dependency

setup.py

@@ -9,11 +9,14 @@ setup(name='config-spec',
       license='MIT',
       packages=['configspec'],
       install_requires=[
-          "toml",
           "preserve@git+https://git.distreon.net/novirium/python-preserve.git"
       ],
       extras_require={
+          'manager': [
+              'toml',
+          ],
           'dev': [
+              'toml',
               'autopep8',
               'pytest',
               'pytest-flake8',
@@ -22,6 +25,7 @@ setup(name='config-spec',
               'tox'
           ],
           'test': [
+              'toml',
               'pytest',
               'pytest-flake8'
           ]