Add InnoSetup make Target

* [x] I have read the [contribution documentation](https://github.com/electron-userland/electron-forge/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project follows, as appropriate.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

**Please describe your issue:**
I'd like to see some alternative make targets on Windows, Squirrel's windows installer does not offer the user feedback or customizable install I am looking to offer so at the moment I am using `forge` to package and InnoSetup to create installer [see this repo for the setup](https://github.com/ValenceEncoder/Valence.git). The end product installer is a more user-friendly and familiar experience than Squirrel currently offers. It's also got some easy to integrate toolchain stuff like a CLI and some [node wrappers](https://www.npmjs.com/package/innosetup-compiler)

If this is something that you think would benefit `electron-forge`, I'd be happy to work on it but would be good to get some pointers on where in the codebase to start.

If you don't think InnoSetup is suitable for `electron-forge` please feel free to close this issue.