download: Infer track and exercise slug from CWD

If I'm in the `exercism/c` directory and invoke:

    exercism download --exercise hello-world

I would like `--track c` to be inferred.

If I'm in the `exercism/c/hello-world` directory and invoke without
arguments:

    exercism download

I would like both `--track c` and `--exercise hello-world` to be
inferred.

cmd/download.go

@@ -50,6 +50,27 @@ Download other people's solutions by providing the UUID.
 	},
 }
 
+func componentsAfterWorkspace(workspace string, n int) ([]string, bool) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return nil, false
+	}
+
+	if !strings.HasPrefix(cwd, workspace) {
+		return nil, false
+	}
+	remaining := cwd[len(workspace):]
+	if remaining[0] == '/' {
+		remaining = remaining[1:]
+	}
+
+	components := strings.Split(remaining, "/")
+	if n <= len(components) {
+		return components[:n], true
+	}
+	return nil, false
+}
+
 func runDownload(cfg config.Config, flags *pflag.FlagSet, args []string) error {
 	usrCfg := cfg.UserViperConfig
 	if err := validateUserConfig(usrCfg); err != nil {
@@ -162,6 +183,19 @@ func newDownload(flags *pflag.FlagSet, usrCfg *viper.Viper) (*download, error) {
 	d.apibaseurl = usrCfg.GetString("apibaseurl")
 	d.workspace = usrCfg.GetString("workspace")
 
+	if d.uuid == "" {
+		if d.slug == "" {
+			if components, ok := componentsAfterWorkspace(d.workspace, 2); ok {
+				d.slug = components[1]
+			}
+		}
+		if d.track == "" && d.team == "" {
+			if components, ok := componentsAfterWorkspace(d.workspace, 1); ok {
+				d.track = components[0]
+			}
+		}
+	}
+
 	if err = d.needsSlugXorUUID(); err != nil {
 		return nil, err
 	}