Expand environment variables on "dir", "sources" and "generates"

So a path like this works: $GOPATH/src/github.com/go-task/task

Allowing of "~" was also implemented. See #74 and baac067a1a

Fixes #116

Taskfile.yml

@@ -13,6 +13,7 @@ vars:
     ./internal/compiler/v2
     ./internal/execext
     ./internal/logger
+    ./internal/osext
     ./internal/output
     ./internal/status
     ./internal/taskfile