Show a progress bar when downloading the index

Right now it seems to freeze forever (it takes about 5 minutes to download on my internet connection):
```
2021/09/29 09:50:47 [INFO] rustwide::cmd: [stderr]     Updating crates.io index
2021/09/29 09:50:47 [INFO] rustwide::cmd: [stderr]      Ignored package `git-credential-null v1.0.1` is already installed, use --force to override
^C
```