Implement getBBox

[nicholascioli]

I'd like to programmatically generate layouts using this library and need to be able to get the size of elements in order to resize backgrounds based on their contents.

This is defined as getBBox in the standard.

If this seems like something that is outside the scope of this project then I could try hacking together the algorithm using the standard as reference.

[IvanUkhov]

Thank you for the note! Well, if I was planning to implement it anyway, I’d open a pull request here to see how it fits into the package. If it does, it will be merged. Otherwise, one would have a working code to be placed elsewhere.