Support custom transport

[altjz]

Hi, I really love the transport design, I used to write a lot scripts but now just a few config. Thanks for providing such awesome tool.

Many web cloud services besides AWS s3 provide sdk for node.js. And I know it is not possible to cover all transport services.
So I wonder if electron-simple-publisher could support extend custom transport.

For example, I write a new transport JS file same as lib/transport/, then use it in publisher.json.

Furthermore, I could create a custom transport and make it a individual module, then other one just install this module and config it in publisher.json.

[megahertz]

Hi. You can set the transport.module option to:

• npm package name
• '{cwd}/path-to-local-transport.js'. the {cwd} template will be replaced by the current working directory (usual project's root)