Allow pre-defined std input

[nkitsaini]

If there was a way to define std-input content at start then we wouldn't need hanger server for some of the cases. My specific case is creating an online environment to practice algorithmic programming questions, in this case user does not need interactive input. Defining an input in text-area before the run should work.

brython.runCode(code, std_input_text) // now the runner does not block on input, it either takes it from `std_input_text` or EOF.

By the way, I also didn't get why we can't do the whole input dance locally. But I'll look more into it to understand the problem.