Port or re-implement HTML export

[mattkimmel]

For the initial version of this lib, the one thing I didn't port from https://github.com/mak-it/bin_packing is the HTML export. @mak-it uses Ruby's ERB engine for this, but Crystal's ECR engine is significantly more limited. Maybe I'm missing a way to use the original template with ECR, but it also might be worth re-implementing this using a different templating lib (or just re-implementing from scratch). It would be cool to be able to use the original templates, to make it easier to pull updates from the Ruby project.