D: Add remaining keywords, rdmd in execute command

I added the remaining keywords that weren't present before. See this:
http://dlang.org/lex.html

Additionally, I also added rdmd in execute which enables the developer to execute D program on fly.
@codebrainz In D, string can't be used as a variable unlike C++, it is imported automatically (builtin), so it make sense to make them highlighted. @elextr Again, like I said earlier maybe they should be highlighted as secondary instead?