Use svgo as peerDependency

[strarsis]

IT would be much better if this imagemin plugin could use svgo as a peerDependency.
Currently it uses svgo as a dependency, which prevents overriding it with a package of different version.
This will also fix #41.

Until svgo is used as a peerDependency, some other mechanism has to be used to force a specific svgo version:
https://stackoverflow.com/questions/15806152/how-do-i-override-nested-npm-dependency-versions

[strarsis]

This is caused by an issue with https://github.com/sindresorhus/gulp-imagemin.