invoke docs: open browser with built docs, and take port from env

tasks.py

@@ -10,10 +10,11 @@
 import glob
 import shutil
 import subprocess
+import webbrowser
 
 from invoke import task
 
-SPHINX_AUTOBUILD_PORT = 8340
+SPHINX_AUTOBUILD_PORT = int(os.environ.get('SPHINX_AUTOBUILD_PORT', '8340'))
 
 
 def watchdog_pid(ctx):
@@ -27,8 +28,8 @@ def watchdog_pid(ctx):
     return pid
 
 
-@task
-def docs(ctx):
+@task(help={'open-tab': "Open docs in new browser tab after initial build"})
+def docs(ctx, open_tab=False):
     """Start watchdog to build the Sphinx docs."""
     build_dir = 'docs/_build'
     index_html = build_dir + '/html/index.html'
@@ -51,7 +52,10 @@ def docs(ctx):
             ctx.run("touch docs/index.rst")
             ctx.run('ps {}'.format(pid), pty=False)
             url = 'http://localhost:{port:d}/'.format(port=SPHINX_AUTOBUILD_PORT)
-            print("\n*** Open '{}' in your browser...".format(url))
+            if open_tab:
+                webbrowser.open_new_tab(url)
+            else:
+                print("\n*** Open '{}' in your browser...".format(url))
             break