feat: custom comment symbol

[Markkop]

Hey, there Yingye. I needed to use custom multi line symbol in a project and adapted your plugin to do this by passing an array as arguments like banner(['My script\nVersion <%= pkg.version %>', '#']) with the string as first argument and the symbol as second. I've updated the readme and made a test for it. If you review and approve this request, I can change my projects dependency to use your repository instead the one I've forked. :rocket: