Removed get_ssid() from being the default for --ssid, fixes #41

The issues is that `get_ssid()` gets executed even though --ssid has not
been called. This must be because it is the default value for the --ssid
argument.

By setting `get_ssid()` as the default value later in the code prevents
the dependency checking for Network Manager on Linux whenever a user
runs `wifi-password --help` to view the usage.

wifi_password/wifi_password.py

@@ -95,14 +95,17 @@ def main():
     parser = argparse.ArgumentParser(usage='%(prog)s [options]')
     parser.add_argument('--qrcode', "-q", action="store_true", default=False, help="Generate a QR code")
     parser.add_argument('--image', "-i", action="store_true", default=False, help="Create the QR code as image instead of showing it on the terminal (must be used along with --qrcode)")
-    parser.add_argument('--ssid', "-s", default=get_ssid(), help="Specify a SSID that you have previously connected to")
+    parser.add_argument('--ssid', "-s", help="Specify a SSID that you have previously connected to")
     parser.add_argument('--version', action="store_true", help="Show version number")
     args = parser.parse_args()
 
     if args.version:
         print(__version__)
         sys.exit()
 
+    if args.ssid is None:
+        args.ssid = get_ssid()
+
     password = get_password(args.ssid)
 
     if args.qrcode: