Remove Node-Canvas dependency but keep NodeJS support

[jamescryer]

The Node-Canvas dependency is a problem for usage, especially on Windows. Other libraries are available, such as https://github.com/joshmarinacci/node-pureimage

I think it would be best if Resemble can be used with Node-canvas or other libraries like pureimage without an explicit dependency. The documentation should show examples of integration with each.

This would be a breaking change.

[FoxxMD]

There have been major updates to pureimage since this was suggested. I think it may implement all the APIs required for canvas that resemble uses at this point? Might just be a drop-in replacement.