Streaming has long since been added. There is an app-wide setting to download new episodes on wifi, download on wifi and cellular, or stream. You cannot configure this per-podcast.
On the list of all current downloads in progress, you can tap a button to convert them all to stream instead of download.
On an individual podcast, you can go to the list of All Episodes and tap once on an episode to fetch it manually. If Overcast is set to download new episodes, this will place the episode in the download queue. However, you can tap the episode once more and it will be switched into streaming mode and begin playing immediately.