Make CLI arguments accessible in zxpy programs

[tusharsadhwani]

Allow passing arguments to CLI applications made with zxpy, by appending them after a --.

For example:

#!/usr/bin/env zxpy

import sys
print("Argv is:", sys.argv)

~"echo output: $1 $2 $3"

$ ./test.py           
Argv is: ['/bin/sh']
output:

$ ./test.py -- abc def
Argv is: ['/bin/sh', 'abc', 'def']
output: abc def

Resolves #50

[tusharsadhwani]

@digitalsignalperson can you check that the bahaviour specified in the PR description is what would be expected?