The fetch command now supports multiple arguments, recursive fetch, and is project-aware

Previously, `dub fetch` could only fetch a single package,
and was working independently of the working directory.

With this release, support for multiple packages have
been added, such that the following is now possible:
---
$ dub fetch vibe-d@0.9.0 vibe-d@0.9.1 vibe-d@0.9.2
---

When called with no argument, `dub fetch` used to error out.
However, it will now attempt to fetch dependencies for the
current project, if any exists.

Finally, when fetching a package, it might be useful to fetch
all its dependencies. This is done automatically for projects,
and can now be done for direct fetch as well:
---
$ dub fetch --recursive vibe-d@0.9.0 vibe-d@0.9.1
---