Download: create directory before writing file

A recent change to exercism [1] allows it to preserve directory
structures in submissions. However, `exercism download` currently cannot
handle such submissions. For example, a submission with `dir/file.txt`
would attempt to write `dir/file.txt` without `dir` existing, and fail.

To fix this, we create any containing directories before writing files.

Fixes #277

[1] exercism/exercism@2dc984d

Author: petertseng

cmd/download.go

@@ -46,6 +46,10 @@ func Download(ctx *cli.Context) {
 
 	for name, contents := range submission.SolutionFiles {
 		filename := strings.TrimPrefix(name, strings.ToLower("/"+submission.TrackID+"/"+submission.Slug+"/"))
+		dir := filepath.Dir(filename)
+		if err := os.MkdirAll(fmt.Sprintf("%s/%s", path, dir), 0755); err != nil {
+			log.Fatal(err)
+		}
 		if err := ioutil.WriteFile(fmt.Sprintf("%s/%s", path, filename), []byte(contents), 0644); err != nil {
 			log.Fatalf("Unable to write file %s: %s", name, err)
 		}