Add support for custom anchors for PNGs and SVGs

[Ninjananas]

Motivation

Hi Andy!

First of all, I'd like to thank you for Squib, it's a great tool that has been a HUGE help in my board game project.

As some people have pointed before in #356 and #238 for example, having more flexible ways to position images would be super cool, and I too need it for my project.

I have not implemented all the ideas in #238, I have only added an anchor: argument to the ScaleBox (which is used by the svg and png dsl methods).

I made sure that my changes are retro-compatible. Existing codebases should not be affected at all by these changes if they don't use the new features.

I have...

• Written a sample in _samples?
  In fact I updated the samples/images/_images.rb sample to add an example of images rotating around different anchors
• Updated documentation in the docs/ folder?
• Linked this PR to an issue?
• Written any automated tests?
  I have written basic unit tests to check the anchor: argument validation
• Refactored your code to be maintainable?
  I tried to make minimal changes and follow the original programming style of the project, but I don't have much experience in Ruby so if you have any suggestions I'm eager to hear them!

I have not...

• Written extensive tests, as I don't know how to write tests that check that the rendering is correct (I manually tested using sanity though)
• Implemented anchor: for text or other methods. (I updated the ScaleBox class which is only used by svg and png)