Show burndown chart for all milestones

[radekstepan]

See discussion here: #43

[nathan-at-least]

Bump. This feature, along with #116 are two requirements my project needs beyond what this project already provides. It's so close. Nice work!

It would be cool to show vertical lines for start/end dates for each milestone, with the chart encompassing the widest range. If those don't line up or make sense, it's up to the user to adjust dates.

[radekstepan]

I can show a chart for issues in all milestones in a project starting with earliest milestone open date (or issue close date) and ending with the last due date (or today). Showing vertical lines would probably just confuse things since until we solve #44 there is no way to future-date milestone start dates.

[radekstepan]

OK implemented, please test.