Add `company-dcd` package that uses DCD (a completion daemon for D) to
provide better IDE features:
- Better code completion.
- Jump to definition.
- Search symbol references.
Helm seems to treat "!" specially in pattern matching, so having a ! in
the pattern string when traversing directories is problematic. This
change fixes#2737, because as far as I can tell "+" has no special
meaning in a helm pattern.
Of course, we can choose a different character, but I'm fond of "+" as
representing "more layers here".
