Implemented basic build platform detection and extended the possibilities for platform specific fields in package.json.

Added the possibility to specify "dflags-xyz" in addition to "dflags" for platform specific flags. ...

Can be one of:
 - dflags
 - dflags-platform
 - dflags-architecture
 - dflags-platform-architecture

where platform is "windows", "linux", "posix" etc. and architecture "x86", "x86_64", "mips" etc. (the lowercase version of the predefined D versions)

Note that the platform is currently hard-coded to windows-x86

Initial commit (more or less the same code as used for vibe.d's vpm).