Feature Request: progress bar and spinner components

[lucatrv]

Please add progress bar and spinner components, similarly to interact.

[bsutton]

These are on my todo list but I've got a few other major pieces that I'm working on.

If you would be interested in submitting a PR it would be well received :D

[lucatrv]

I'm still learning Dart so I would not be able to contribute unfortunately... thank you for your effort.
I also wanted to point you out the interact project which has already interesting implementations (not only progress bars and spinners, but also other useful components). That project appears not very active lately (last commit more than one year ago), so maybe the developer would agree to marge efforts into dcli which seems very promising...