support specifying --d-versions ...

Usually you should prefer using configurations and build types, but
sometimes, especially when wrapped in other build tools or when
supporting e.g. different target platforms on the same target
architecture and configuration, you may want to manually manage certain
versions.

Add support for coverage at compile-time (CTFE)

Add zsh completion script ...

Currently supports the main dub commands: build, init, run, test, clean, dustmite...
Reads the local packages from dub whenever possible.
A few improvements to dub will be necessary to make it completely useful (e.g. display configurations).