Add output-path, configs, builds to dub describe

[WebFreak001]

Also contains default-config and default-build

Extends the describe command from only being build options to also custom fields like this. Configs and builds was previously only exposed in "interactive" TUI, now as machine parsable format.

output-path is for building the full path including platform specific extensions for where a file is placed. This can be used to check for example after a build if a file has changed or do some extra work on it.

Needs tests right now

[WebFreak001]

found bugs:

• when running with --build=unittest, the output-path is not the unittest executable

• in arsd-official running dub describe :eventloop --config=library --data=output-path --data-list it segfaults (target type is sourceLibrary)

  using other data types it says Target 'arsd-official:eventloop' doesn't exist. Is the target type set to "none" in the package recipe?

note:

• silently fails with exit code 2 if using a --config and the platform specifier does not match (was also earlier behavior) - makes sense because on different platforms the configurations may result in different specifications - and exiting with failure is a good way to say invalid config for this platform. There should be some output to stderr though.