Feature request: Add filter for [portfolio]-Shortcode output

Hello there!

It would be nice to have a filter for the shortcode function output of [portfolio]. I was searching the jetpack plugin files, but could not find any filter.. so after Jeremy's suggestion I figured I'd open a github issue for this matter. The related wordpress.org-support thread is the following: https://wordpress.org/support/topic/customizing-jetpack-portfolio-shortcode

I'd like to control the html output of the shortcode to be able to add a div if needed or implement a custom field I might have registered for my portfolio post types. For example I set up a custom field which allows me to insert a client logo. I would like to have this client logo (added via custom field in portfolio cpt) to be displayed with the portfolio item's title. I would like to be able to control where exactly this custom field shows up and which css classes are spread out.

A filter would allow such for the [portfolio] output.

Thanks