Allow Roadmap Planner JSON data to support colors per-bar

We really like the confluence roadmap planner. Although I realize that it does not have an official API, we still found it reasonably straightforward to extract data from JIRA and our other tracking systems and automatically create custom roadmaps that help us understand how our work rolls up across 30 scrum teams. (Note that the JIRA product roadmap tool will not work for us because we also need to display custom operational data, for which we use confluence's chart functionality to get it all into one place.)

In order to understand how the work rolls up, we define two levels of epic - larger ones that span across teams and smaller ones that belong to one team. We really would like to be able to show the larger and smaller epics together in the same lane but in different colors so that someone viewing the page is clear that these are different things. Different colors would also help us convey other levels of information - for example, the product or target release for an epic, or which team is actually working on it.

Note that this is not a request to support individual bar colors in the UI. I realize that would likely end up being more work and introducing potential usability problems. This request is just to support optionally including a color as part of the JSON data for the bar and displaying the bar in that color if it is present.

The lane is already being used to group related items together, so we would prefer not to have to separate these issues into different lanes.