[IMP] parser: add support for custom directives

This commit adds the support for custom directives. To use the
custom directive, an Object of functions needs to be configured on
the owl APP:
```js
 new App(..., {
    customDirectives: {
     test_directive: function (el, value) {
            el.setAttribute("t-on-click", value);
            return el;
       }
   }
  });
```
The functions will be called when a custom directive with the name of the
function is found. The original element will be replaced with the one
returned by the function.
This :
```xml
<div t-custom-test_directive="click" />
```
will be replace by :
```xml
<div t-on-click="value"/>
```

issue : https://github.com/odoo/owl/issues/1650

doc/reference/app.md

@@ -66,6 +66,8 @@ The `config` object is an object with some of the following keys:
   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).
+- **`customDirectives (object)`**: if given, the corresponding function on the object will be called
+  on the template custom directives: `t-custom-*` (see [Custom Directives](templates.md#custom-directives)).
 
 ## `mount` helper