Write files with extension `.arrows` in Arrow IPC stream format

[ianmcook]

databow currently recognizes .arrow as the extension for Arrow IPC file format (with the footer). Let's please make it also recognize .arrows as the extension for Arrow IPC stream format (without the footer) and be able to write that to disk.

[ianmcook]

Let's also explain this in the databow docs. It's a bit confusing since both are files but one is called "stream format".