Automatically open browser when Invoke task starts web server (#2764)

When the `serve` and `livereload` targets are invoked, a web browser will be
automatically opened, pointing to the locally-served website.

If no web browser can be found by the module, the `open()` call returns
`False`, but no exception is raised. This means that it is still possible
to call livereload on a remote machine and access it without any error
being triggered.

Signed-off-by: Romain Porte <microjoe@microjoe.org>

pelican/tools/templates/tasks.py.jinja2

@@ -13,6 +13,7 @@ from pelican import main as pelican_main
 from pelican.server import ComplexHTTPRequestHandler, RootedHTTPServer
 from pelican.settings import DEFAULT_CONFIG, get_settings_from_file
 
+OPEN_BROWSER_ON_SERVE = True
 SETTINGS_FILE_BASE = 'pelicanconf.py'
 SETTINGS = {}
 SETTINGS.update(DEFAULT_CONFIG)
@@ -81,6 +82,11 @@ def serve(c):
         (CONFIG['host'], CONFIG['port']),
         ComplexHTTPRequestHandler)
 
+    if OPEN_BROWSER_ON_SERVE:
+        # Open site in default browser
+        import webbrowser
+        webbrowser.open("http://{host}:{port}".format(**CONFIG))
+
     sys.stderr.write('Serving at {host}:{port} ...\n'.format(**CONFIG))
     server.serve_forever()
 
@@ -124,6 +130,12 @@ def livereload(c):
 
     for glob in watched_globs:
         server.watch(glob, cached_build)
+
+    if OPEN_BROWSER_ON_SERVE:
+        # Open site in default browser
+        import webbrowser
+        webbrowser.open("http://{host}:{port}".format(**CONFIG))
+
     server.serve(host=CONFIG['host'], port=CONFIG['port'], root=CONFIG['deploy_path'])
 
 {% if cloudfiles %}