Improve App configuration UX with `Tmpl`

[cusma]

I've played a bit with Beaker v1 and I would like to report a feedback.

Now that the tool is instance-based (instead of class-based), working with Tmpl seems the best way to deal with parametric configurations of App constants (previously I used the __init__ of the App Class to have instance-specific self constants).

I think that Beaker could improve the UX of working with Tmpl introducing a class, or a helper, that handles the Tmpl constants facilitating the replace of TMPL_ placeholders with actual values in the TEAL code, avoiding "string replace" boilerplate.