make callback split by newline

[clouds56]

This PR keeps backwards compatibility as when nothing passed in as eol, it keeps considering every input as a single line. While it could benefit some spawn function (at least on macOS) that would split input buffer when a line longer than 8192 characters. i.e. make the following code actually work without handle '\n' manually

const child = spawn(cmd, args)
child.stdout.on('data', runner.callback('\n'))