Allow window() to include latest data point

[phemmer]

It would be really useful if window() could emit the latest data point instead of all but the latest. The use case is wanting to obtain the last X seconds worth of data each time a new data point comes in. In this scenario we are emitting the window on every incoming data point, thus we don't need to hold back the latest data point so that we can make sure it fits inside a window.

For example, consider input data with the following times:

00:00:01
00:00:02
00:00:03
00:00:05
00:00:09

With a window size of 3s, the resulting emitted points would be:

00:00:01: [00:00:01]
00:00:02: [00:00:01, 00:00:02]
00:00:03: [00:00:01, 00:00:02, 00:00:03]
00:00:05: [00:00:03, 00:00:05]
00:00:09: [00:00:09]

This mode could be enabled by having a 0 value every(). Such as .every(0s), .every(0u), etc.

[nathanielc]

@phemmer I like your suggestion of using the 0 value for the every property. There may be some weird edge cases where this doesn't work well, but at first glance it seems like a reasonable request.