Support importing a svg into the SVG object
Would be really cool if we could get functionality that allowed us to take an already existing svg (a file, for example) and import it into svgo and modify it! 

This would be really helpful when working with large preexisting svgs and adding dynamic content on top of it :)

Here how I envision it to be used:
```go
buf, err := os.OpenFile("my.svg")
if err != nil {
  return err
}
canvas, err := svg.Import(buf, os.Stdout)
if err != nil {
  return err
}
canvas.DoAwesomeThings(...)
// ...
canvas.End() // my.svg with awesome things done to it printed to stdout
```