Add an option to open() the file descriptor

[avitkauskas]

This PR adds an option (overload) to the open() builtin function to allow opening the file descriptors (as in python).

Useful when you want to read from the standard input:

# app.codon
for line in open(0):
    print(line)

then:

codon run app.codon < input.txt

[cla-bot]

Thank you for your pull request. We require contributors to agree to our Contributor License Agreement (https://exaloop.io/cla.txt), and we don't have @avitkauskas on file. In order for us to review and merge your code, please email info@exaloop.io to get yourself added.

[avitkauskas]

This PR would allow to close #313