Custom outDir

Hi there!

It looks [this code](https://github.com/electron-userland/electron-forge/blob/master/src/api/make.js#L43) doesn't work correctly. The second expression only. I think it happens because you don't pass this options [here](https://github.com/electron-userland/electron-forge/blob/master/src/electron-forge-make.js#L29). It would be better to pass options from `config.forge` here too.