[IMP] template set config: getTemplate function

A template set can now obtain a template by calling the function
getTemplate (if any) received in the initial config.
The returned value can be an Element instance, a template string, a
function (i.e. a compiled template), or null. In the last case, owl will
look into the template set rawTemplates object.

doc/reference/app.md

@@ -61,6 +61,8 @@ The `config` object is an object with some of the following keys:
   templates (see [translations](translations.md))
 - **`templates (string | xml document)`**: all the templates that will be used by
   the components created by the application.
+- **`getTemplate ((s: string) => Element | Function | string | void)`**: a function that will be called by owl when it
+  needs a template. If undefined is returned, owl looks into the app templates.
 - **`warnIfNoStaticProps (boolean, default=false)`**: if true, Owl will log a warning
   whenever it encounters a component that does not provide a [static props description](props.md#props-validation).