[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

src/index.ts

@@ -12,5 +12,6 @@ TemplateSet.prototype._compileTemplate = function _compileTemplate(
     dev: this.dev,
     translateFn: this.translateFn,
     translatableAttributes: this.translatableAttributes,
+    customDirectives: this.customDirectives,
   });
 };