Add abiulity to preserve existing viewBox

This release introduces usage of SVGO plugin params.
The first supported "param" is the overwrite which defaults totrue.
If set to false, svgoAddViewbox will not overwrite any existing viewBox attribute on your input SVGs.
Usage:

plugins: [
  // ... more plugins
  {
    ...addViewBox,
    params: {
      overwrite: false
    }
  }
  // ... more plugins
];