Added experimental build settings to filter applicable version identifiers

Those settings allow to filter version identifiers and debug version identifiers
used during building a package. This can improve the build cache efficiency for
libraries that are used by multiple applications with different version
identifiers.

dub.json:
```json
"-versionFilters": ["Have_vibe_d"]
"-versionFilters-posix": ["UseUnixSockets", "UseMMap"]
```
dub.sdl:
```sdl
x:versionFilters "Have_vibe_d"
x:versionFilters "UseUnixSockets" "UseMMap" platform="posix"
```