support progress bars for iroh add command

[faassen]

• initial phase where it scans directories, with scrollbar. (if adding a directory)
• then per file, an individual scrollbar. This wants to use some kind of multi scrollbar approach
• in the API we want to support getting progress information if we opt into it

We don't want to see the scrollbars if we're executing this in a script.

This looks like a good place to start:

https://docs.rs/indicatif/latest/indicatif/

I split off this issue from #179 as it's complicated enough to deserve its own.

[faassen]

Ah, #183 is already there, so will go there.