Added 'previous' and 'diff' modules

[jwoudenberg]

I regularly want to create a stream that responds to changes in values in another stream. To help with that, I added two modules that I think might be interesting to others.

flyd-previous takes a stream and returns a stream that's always one value behind the original stream.

flyd-diff uses flyd-previous to provide a variant of map, in which the mapper function is called with the previous and new value of the stream.

Additionally, I took the liberty of adding a basic .editorconfig file, which I think is a great help for contributors, but if it's not liked just tell me and I'll take it out again :).

I'm looking forward to all feedback!

[paldepind]

This looks very good! These are nice modules in my opinion 👍

I have one concern though. From looking at the source code it appears that a stream created with previous will always initially emit an undefined when its source stream receives its first value? That does not seem ideal to me.

[jwoudenberg]

Thanks! Hmm, I see your point. It would make more sense if it starts streaming once the second value arrives, right? I'll change that.

[paldepind]

Yes, exactly :)

[jwoudenberg]

I changed the previous module to start streaming once the source stream receives its second value. Of course the diff function exhibits the same behavior.

Let me know if you have any more feedback :).

[paldepind]

Really great! Thank you for this contribution 😄

[jwoudenberg]

Thanks!