show progress of downloads

[ekcasey]

Motivation

Some buildpacks are large and therefore a buildpack download during pack create-builder can take a long time. As a user I want visual feedback about the progress of a download so that I have confidence pack is still working have some idea how long I will have to wait for the download to complete.

Suggested UX

When pack is downloading an asset (e.g. a buildpack, or a lifecycle archive)
Then I should see a progress bar that updates as the download proceeds

Example:

$pack create-builder
...
Downloading from "https://example.com/my/buildpack.tgz"
[=======>          ] 50%
...

When the download is complete
Then the progress bar should be replaced with the message Download Complete