Allow building SVG badges or add a hook so one can get at data to do so

[brettz9]

Hi,

During the process of getting size info, I'd like to be able to use that info, e.g., with badge-up, to build badge(s) which can be added to one's README to show the current file size of one's dist files.

I was wondering if you wanted this support built-in (badge-up only adds 16KB, it being an SVG generator rather than a PNG generator), or at least if you could allow users to supply a hook function that was given the generated size info, so they could make their own badge (or whatever).

Thanks for the excellent tool!

(FWIW, we're already using this approach to build local badges for Mocha, license types, and coverage, and filesize would be a great addition.)