Support streams for downloading envelopes

[jchull]

We would like to be able to transfer an envelope without storing the entire contents in memory, streams have been around for a while and provide this functionality.

Why streams
Streams basically provide two major advantages compared to other data handling methods:

Memory efficiency: you don’t need to load large amounts of data in memory before you are able to process it

Time efficiency: it takes significantly less time to start processing data as soon as you have it, rather 
than having to wait with processing until the entire payload has been transmitted

More on streams: https://nodesource.com/blog/understanding-streams-in-nodejs/

[HobbyProjects]

Thank you for the suggestion. We'l consider this for future releases.