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 nthComponentAfterWorkspace(workspace string, n int) (string, bool) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return "", false
+	}
+
+	if !strings.HasPrefix(cwd, workspace) {
+		return "", false
+	}
+	remaining := cwd[len(workspace):]
+	if remaining[0] == '/' {
+		remaining = remaining[1:]
+	}
+
+	components := strings.Split(remaining, "/")
+	if n < len(components) {
+		return components[n], true
+	}
+	return "", 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 slug, ok := nthComponentAfterWorkspace(d.workspace, 1); ok {
+				d.slug = slug
+			}
+		}
+		if d.track == "" && d.team == "" {
+			if track, ok := nthComponentAfterWorkspace(d.workspace, 0); ok {
+				d.track = track
+			}
+		}
+	}
+
 	if err = d.needsSlugXorUUID(); err != nil {
 		return nil, err
 	}

cmd/download_test.go

@@ -77,6 +77,63 @@ func TestDownloadWithoutFlags(t *testing.T) {
 	}
 }
 
+func TestDownloadInferringFlags(t *testing.T) {
+	co := newCapturedOutput()
+	co.override()
+	defer co.reset()
+
+	originalCwd, err := os.Getwd()
+	defer os.Chdir(originalCwd)
+	assert.NoError(t, err)
+
+	testCases := []struct {
+		cwd   string
+		flags map[string]string
+	}{
+		{
+			cwd:   "bogus-track",
+			flags: map[string]string{"exercise": "bogus-exercise"},
+		},
+		{
+			cwd:   "bogus-track/bogus-exercise",
+			flags: nil,
+		},
+	}
+
+	for _, tc := range testCases {
+		tmpDir, err := ioutil.TempDir("", "download-infer")
+		defer os.RemoveAll(tmpDir)
+		assert.NoError(t, err)
+
+		subdir := filepath.Join(tmpDir, tc.cwd)
+		err = os.MkdirAll(subdir, 0755)
+		assert.NoError(t, err)
+		os.Chdir(subdir)
+
+		ts := fakeDownloadServer(strconv.FormatBool(true), "")
+		defer ts.Close()
+
+		v := viper.New()
+		v.Set("workspace", tmpDir)
+		v.Set("apibaseurl", ts.URL)
+		v.Set("token", "abc123")
+
+		cfg := config.Config{
+			UserViperConfig: v,
+		}
+		flags := pflag.NewFlagSet("fake", pflag.PanicOnError)
+		setupDownloadFlags(flags)
+		for name, value := range tc.flags {
+			flags.Set(name, value)
+		}
+
+		err = runDownload(cfg, flags, []string{})
+		assert.NoError(t, err)
+
+		assertDownloadedCorrectFiles(t, tmpDir)
+	}
+}
+
 func TestSolutionFile(t *testing.T) {
 	testCases := []struct {
 		name, file, expectedPath, expectedURL string