Added support for environment variables to use compilation and run(or test) option to the $(LINK2 https://dub.pm/settings, dub settings file) and dub.json/dub.sdl.

The following items have been added to the dub setting file: `defaultEnvironments`, `defaultBuildEnvironments`, `defaultRunEnvironments`, `defaultPreGenerateEnvironments`, `defaultPostGenerateEnvironments`, `defaultPreBuildEnvironments`, `defaultPostBuildEnvironments`, `defaultPreRunEnvironments`, `defaultPostRunEnvironments`.
They are used when there are no project-specific settings.

```
{
    "defaultEnvironments": {
        "VAR": "Foo"
    }
}
```

The following items are available in each project-specific setting: `environments`, `buildEnvironments`, `runEnvironments`, `preGenerateEnvironments`, `postGenerateEnvironments`, `preBuildEnvironments`, `postBuildEnvironments`, `preRunEnvironments`, `postRunEnvironments`.

In JSON
```
{
    "environments": {
        "VAR": "Foo"
    }
}
```

In SDL
```
environments "VAR" "Foo"
```