Preserve line numbers

[cspotcode]

If shebang loader leaves a blank line where the shebang used to be, then line numbers will not change. This means sourcemaps -- if they exist -- will still be valid.

[cspotcode]

Along with this, shebang-loader should pass through a sourcemap if it's passed one. I don't think webpack does this automatically on behalf of the loader, though I could be wrong about that.