Suggestion: support for SteamPipe custom folders in fastdl module

I would like to suggest adding support for SteamPipe custom folders in the fastdl module. Details on this can be found here : https://support.steampowered.com/kb_article.php?ref=7388-QPFN-2491

Why do this? Well for one it's an official feature that probably most source servers are using (at least in tf2). Secondly there's no point in wasting time and resources by copying the stock maps from the default directories. In TF2 for example the fastdl command copies and compresses around 4 gigs from just the stock files alone. And it does this every time the command is executed, which leads me to my second suggestion.

I think it would be beneficial to run checksums on files to determine whether or not they should be copied and compressed. There's no point in re-compressing the same files over and over right?

So for example, rather than doing tf/maps, you would be checking tf/custom//maps

Just some ideas.