Allow spaces in shell executables

[greenfork]

Closes #28

[bakpakin]

I get why you want to add this but it is a bit awful. At least add a comment explaining why this is there and then we can merge it.

[greenfork]

I share your sentiment, this is awful. There are some alternatives I have considered:

• Specifying C compiler via declare - I didn't find any options in declare.janet.
• Wrapping zig cc in a script - I couldn't make that work with os/execute, it probably expects an executable binary file as the first argument. There's an option to compile a program which passes all arguments to zig cc - probably it is going to work but looks like even more awful thing to do compared to this pull request.
• Using string/split on all arguments (def args (mapcat |(string/split " " $) args)) - looks much less awful. Also it helps with specifying several flags in --cflags='-flto -O2'. Otherwise compiler complains Unknown Clang option: '-flto -O2'. But I don't know how safe it is to assume that no options contain spaces, probably there's a case where it is not safe. At the same time to my best knowledge there's no way to specify multiple flags via --cflags.

I'm not in a hurry to merge this and I'm open to suggestions. For now I added the comment you asked and feel free to merge it if you think this is good enough.

[bakpakin]

I'm pretty sure this is fine, I can't imagine any sane, informed person putting spaces in the name of a binary, but we should at least add a comment why this is being done.

[greenfork]

Thanks!