Add support for signing Flatpak builds

[CSstefan]

What version of electron-installer-flatpak are you using?

0.8.0

What version of node and npm are you using?

node 7.7.4
nom 4.1.2

What operating system are you using?

Ubuntu 16.04

Can you compile the example app successfully?

Yes

If not, paste here the output of the following commands:

$ git clone https://github.com/endlessm/electron-installer-flatpak.git
$ cd electron-installer-flatpak/example
$ DEBUG='electron-installer-flatpak' npm run build

What did you do? Please include the configuration you are using for electron-installer-flatpak.

Added "gpg-sign" under electronInstallerFlatpak in my packages.json

What did you expect to happen?

I expected the option to be passed to flatpak build-export to sign the package

What actually happened?

The option does not get passed.

I worked with @anulman to trace the issue to here:

electron-installer-flatpak/src/installer.js

Lines 336 to 339 in d3a7cbd

	}, {
	arch: options.arch,
	bundlePath: dest
	}, function (err) {

Only arch and bundlePath are being passed. Perhaps all the electronInstallerFlatpak should be passed?

[cosimoc]

@CSstefan Thanks for the report! Yeah, I agree that it would be great to have a better way to pass export options along, or at least support the GPG use cases. I'd be happy to accept patches!