Add configuration to dynamically display latest release on Mesos website

When an committer updates the website for a release, there are currently several places they need to reflect the latest version: the homepage's download button, the downloads page, and the getting started page.

It'd be ideal if a committer had a single configuration file for the website that they could update to reflect the latest version #, and these parts of the website would be updated.

In order to complete this, we'd need a proposal for the best way to manage this type of configuration within middleman, and a patch to auto-generate the website in places described above. This may pose issues for the getting started page, which currently renders static markdown files. Proposed solutions are welcome!