Implemented pre/post commands and --build=ddox + support "sourceFiles". ...

Other fixes:
 - fixed class Dub to work with other path than cwd
 - "files" has been deprecated in favor of "sourceFiles"
 - --build=docs also works now
 - Changed the way build flags are parsed (instead of checking every possible field for existence, all fields are iterated and their name is matched)

Renamed all the "vibe" package to "vibecompat" to avoid conflicts.

Added rudimentary support for the --arch= option and moved build type flag generation to a common place.

Added a new "build" generator that directly invokes the compiler. ...

This generator supports all compilers in contrast to the rdmd based one and will later be the default.

Added basic GDC support (still has issues with the -of flag.