[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/templates.md

@@ -18,6 +18,7 @@
   - [Sub Templates](#sub-templates)
   - [Dynamic Sub Templates](#dynamic-sub-templates)
   - [Debugging](#debugging)
+  - [Custom Directives](#custom-directives)
 - [Fragments](#fragments)
 - [Inline templates](#inline-templates)
 - [Rendering svg](#rendering-svg)
@@ -80,6 +81,7 @@ needs. Here is a list of all Owl specific directives:
 | `t-slot`, `t-set-slot`, `t-slot-scope` | [Rendering a slot](slots.md)                                    |
 | `t-model`                              | [Form input bindings](input_bindings.md)                        |
 | `t-tag`                                | [Rendering nodes with dynamic tag name](#dynamic-tag-names)     |
+| `t-custom-*`                           | [Rendering nodes with custom directives](#custom-directives)    |
 
 ## QWeb Template Reference
 
@@ -588,6 +590,35 @@ will stop execution if the browser dev tools are open.
 
 will print 42 to the console.
 
+### Custom Directives
+
+Owl 2 supports the declaration of custom directives. To use them, 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);
+       }
+   }
+  });
+```
+
+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
+modified by the function.
+This :
+
+```xml
+<div t-custom-test_directive="click" />
+```
+
+will be replaced by :
+
+```xml
+<div t-on-click="value"/>
+```
+
 ## Fragments
 
 Owl 2 supports templates with an arbitrary number of root elements, or even just