Tell how many bytes "install" will take

[jidanni]

Regarding the two steps of
https://snapcraft.io/install/authy/debian

The first tells me how much I need to download, and asks if I want to
continue.

The second doesn't.

In fact it is still downloading things as I speak.

No clue how many things or when it will stop.

[jidanni (dan-jacobson)]

(The first is apt(itude), the second is snap.)

[Maciej Borzecki (maciek-borzecki)]

Right, we don't show the download size, only the speed and percentage progress. Maybe we should include that in the progress bar.

As a workaround `snap info` includes the size information of a snap per channel.