Support ffi in dotPsci

[sharkdp]

It would be nice if the dotPsci task would also support the ffi option. It would add

:f path/to/module.js

lines to the .psci file to load the corresponding foreign code when running psci.

[ethul]

Agreed. I will add this in conjunction with the changes for #38

Thanks!

[sharkdp]

Great, thanks!