Add download statistic

[cliffparnitzky]

Adding a download statistic will be fine.

[cliffparnitzky]

Storing will be possible with https://github.com/BugBuster1701/dlstats

A module is needed, which reads the data from a specific document.

[cliffparnitzky]

A complete new extension is needed, because with #31 the postDownload hook will not longer be triggered.

[cliffparnitzky]

Use the Contao "dmsPostDocumentDownload" hook hook to store statistic data.