Support for python -m pelican

Addresses https://github.com/getpelican/pelican/issues/2523

Note: @avaris made a good point that there is no need to replace the existing
module runner for pelican quickstart or the other tools as this can be run
via:

> python -m pelican.tools.pelican_quickstart

pelican/__main__.py

@@ -0,0 +1,10 @@
+"""
+python -m pelican module entry point to run via python -m
+"""
+from __future__ import absolute_import
+
+from . import main
+
+
+if __name__ == '__main__':
+    main()

pelican/tests/test_pelican.py

@@ -263,3 +263,10 @@ class TestPelican(LoggedTestCase):
             count=1,
             msg="Could not process .*parse_error.rst",
             level=logging.ERROR)
+
+    def test_module_load(self):
+        """Test loading via python -m pelican --help displays the help"""
+        output = subprocess.check_output([
+            sys.executable, '-m', 'pelican', '--help'
+        ]).decode('ascii', 'replace')
+        assert 'usage:' in output