[Python bindings] `hl.main()` does not accept arguments

[jiawen]

As the title says, when using the Python bindings with generators, hl.main() does not accept any arguments. It instead reads sys.argv (this was not easy to find!).

Discussed offline with @alexreinking: it should allow argv to be passed in but default to to sys.argv. We may also want to go all in on using kwargs instead.

This is probably easier to implement in the form of a Python wrapper than directly in Pybind11.