option for output path

[ainozaki]

Add option to specify output path

[saza-ku]

Cool!

It would be better to specify the enter path of the output file, instead of the directory of it.

And we would update the docs.

[ainozaki]

Thank you!
I've modified it to specify a file path. The default value remains the same (output will still be directed to "./wasm.o" as before.)

[saza-ku]

Looks good!