Build the HTML docs and open them in a web browser.

def doc_open():
    """Build the HTML docs and open them in a web browser."""
    doc_index = os.path.join(DOCS_DIRECTORY, 'build', 'html', 'index.html')
    if sys.platform == 'darwin':
        # Mac OS X
        subprocess.check_call(['open', doc_index])
    elif sys.platform == 'win32':
        # Windows
        subprocess.check_call(['start', doc_index], shell=True)
    elif sys.platform == 'linux2':
        # All freedesktop-compatible desktops
        subprocess.check_call(['xdg-open', doc_index])
    else:
        print_failure_message(
            "Unsupported platform. Please open `{0}' manually.".format(
                doc_index))