Add argument for printing last result of executed file

[dancek]

For my BQN slack bot, it seems better to put larger amounts of code in a file and execute from there. But I need to print the last evaluation result, so something like this would be useful.

I can rewrite this if you have better ideas of how it should be done, or if you'd rather do it yourself feel free to close this PR.

[dzaima]

It's easy enough to do this manually - bqn -p '•Import"file.bqn"'. The arguments are there just for the most common of the most common use-cases, everything else should be done within BQN code.