Add custom exporter support for cli tool

[MinchukSergei]

Hi. What do you think about add custom exporter like this:

options.exporter = {
	fileExt: argv.customext,
	template: argv.custom
}

and run cli command such as:
free-tex-packer-cli --project packer.ftpp --custom [path to custom.mst file] --customext [any output extension]

[odrick]

At the moment I don't have much time to develop this project. Feature requests are postponed.

[rainbowcreatures]

Or, simply workaround by adding your custom template into a new .mst file under node_modules\free-tex-packer-core\exporters. Also add a definition into list.json, in the same folder.

Done, you can now use your custom exporter.