Introduce a `console` package which encapsulates logic of printing out
to console. This provides flexibility of discarding output in tests.
It also allows us to unify format of printing to console (if we want to).

This fixes #801.

Print out aggregate error of asset upload instead of failing the whole upload process if one of them fails

Detect the asset info to upload via the `--attach` flag.

"The first block of the text"
becomes:
"The first block of text"

The definite article was not needed here.

So we can use flags before and after arguments, and we can group boolean flags, just like git does.

- Use a hash to lookup for commands.
- Add more tests for the command and runner execution.

Change the message only when the upload is done.

Removes duplicated test helper.