Allow setting host in Invoke serve & livereload tasks

Instead of serving on localhost by default with no way to override, the
host can now be configured, allowing both `serve` and `livereload` tasks
to serve output on non-localhost addresses such as `0.0.0.0`.

pelican/tools/templates/tasks.py.jinja2

@@ -39,7 +39,8 @@ CONFIG = {
     'github_pages_branch': '{{github_pages_branch}}',
     'commit_message': "'Publish site on {}'".format(datetime.date.today().isoformat()),
 {% endif %}
-    # Port for `serve`
+    # Host and port for `serve`
+    'host': 'localhost',
     'port': 8000,
 }
 
@@ -67,17 +68,17 @@ def regenerate(c):
 
 @task
 def serve(c):
-    """Serve site at http://localhost:$PORT/ (default port is 8000)"""
+    """Serve site at http://$HOST:$PORT/ (default is localhost:8000)"""
 
     class AddressReuseTCPServer(RootedHTTPServer):
         allow_reuse_address = True
 
     server = AddressReuseTCPServer(
         CONFIG['deploy_path'],
-        ('', CONFIG['port']),
+        (CONFIG['host'], CONFIG['port']),
         ComplexHTTPRequestHandler)
 
-    sys.stderr.write('Serving on port {port} ...\n'.format(**CONFIG))
+    sys.stderr.write('Serving at {host}:{port} ...\n'.format(**CONFIG))
     server.serve_forever()
 
 @task
@@ -111,8 +112,8 @@ def livereload(c):
     for extension in static_file_extensions:
         static_file = '{0}/static/**/*{1}'.format(theme_path, extension)
         server.watch(static_file, lambda: build(c))
-    # Serve output path on configured port
-    server.serve(port=CONFIG['port'], root=CONFIG['deploy_path'])
+    # Serve output path on configured host and port
+    server.serve(host=CONFIG['host'], port=CONFIG['port'], root=CONFIG['deploy_path'])
 
 {% if cloudfiles %}
 @task