This commit:

- Adds support to download all sources concurrently, each in their own
thread, rather than sequentially, in the interests of saving time. It
emits an interim progress report of which sources are still being
downloaded every time a thread completes

- Adds support for an arbitrary number of sources to download other than
just all or one of them

- Adds support for excluding an arbitrary number of sources when
downloading all of them

- fixes a bug where the `--verbose` flag was not being honoured when
downloading all sources.