Accept command-line args in programs

[mihirmanna]

When running a Numbat program from the terminal (e.g., numbat my_program.nbt), it would be great to accept command-line args afterwards (e.g., numbat my_program.nbt 100). In general, it also seems like we are missing a good way to use stdin.

For multiple arguments or arguments with units, it becomes a little tricky to split the arguments correctly--e.g., how to parse

numbat circle_area.nbt 2cm

vs.

numbat circle_area.nbt 2 cm

Although that is something we can probably set a standard for.