WIP: still hacking on the app new command

Finally had to fork godotenv because it strips comments and we need
those to parse length values (e.g. "FOO=v1  # length=10") (or in other
words, motivation to move to the YAML format).

There is a new secret module now, with functionality for dealing with
generation and parsing of secrets.

The final output needs some work and there is also the final step of
implementing the sending of secrets to the docker daemon. Coming Soon
™️.

catalogue/catalogue.go

@@ -103,6 +103,15 @@ func (a App) EnsureVersion(version string) error {
 	return nil
 }
 
+func (a App) LatestVersion() string {
+	var latestVersion string
+	for tag := range a.Versions {
+		// apps.json versions are sorted so the last key is latest
+		latestVersion = tag
+	}
+	return latestVersion
+}
+
 type Name = string
 type AppsCatalogue map[Name]App