Single-command flag for macropython CLI

[Technologicat]

The incantation python -c 'import foo; print(foo.__version__)' is pretty useful for checking library versions.

It would be useful to have this in macropython, too, but currently, it only supports files and an interactive shell.

Also, for macropython the -c flag means something completely different (clean pycache dirs), because I completely forgot about Python's standard -c flag when I implemented the cache cleaner.

Perhaps the best solution is to rename our nonstandard flag to -C, but that must wait for 4.0.