Add ability for comment lines to be ignored

[paulhayes]

Thanks for making such a great library. Incredibly useful. Especially because you have both Serial attachment and a more manual shell.execute(line); features.

There's only one feature that I think would be really useful, and that's the ability to register a comment character like "#", that means a line is skipped over immediately without a command not found message being outputted.

[philj404]

Try this:

Create a "do nothing" command and add it with shell.addCommand(F("#"), doNothing);

If you would rather use a different comment marker, you can change it.

[philj404]

Instead add a "do nothing" command and as suggested above.