Automate image optimisation

(Mergeable version of #123)
You got me there. I blindly copied that code from gulp-imagemin https://github.com/sindresorhus/gulp-imagemin#usage

After some research It looks like 2 SVGO plugin settings are actually recommended:

```
                {
                    removeViewBox: false
                }, {
                    removeUselessStrokeAndFill: false
                }
```

That should avoid any destructive compression on complex paths.