[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

tests/compiler/__snapshots__/t_custom.test.ts.snap

@@ -0,0 +1,29 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`t-custom can use t-custom directive on a node 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div class=\\"my-div\\" block-handler-0=\\"click\\"/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let hdlr1 = [ctx['click'], ctx];
+    return block1([hdlr1]);
+  }
+}"
+`;
+
+exports[`t-custom can use t-custom directive with modifier on a node 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div class=\\"my-div\\" block-handler-0=\\"click\\"/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let hdlr1 = [ctx['click'], ctx];
+    return block1([hdlr1]);
+  }
+}"
+`;

tests/compiler/t_custom.test.ts

@@ -0,0 +1,55 @@
+import { App, Component, xml } from "../../src";
+import { makeTestFixture, snapshotEverything } from "../helpers";
+
+let fixture: HTMLElement;
+
+snapshotEverything();
+
+beforeEach(() => {
+  fixture = makeTestFixture();
+});
+
+describe("t-custom", () => {
+  test("can use t-custom directive on a node", async () => {
+    const steps: string[] = [];
+    class SomeComponent extends Component {
+      static template = xml`<div t-custom-plop="click" class="my-div"/>`;
+      click() {
+        steps.push("clicked");
+      }
+    }
+    const app = new App(SomeComponent, {
+      customDirectives: {
+        plop: (node, value) => {
+          node.setAttribute("t-on-click", value);
+        },
+      },
+    });
+    await app.mount(fixture);
+    expect(fixture.innerHTML).toBe(`<div class="my-div"></div>`);
+    fixture.querySelector("div")!.click();
+    expect(steps).toEqual(["clicked"]);
+  });
+
+  test("can use t-custom directive with modifier on a node", async () => {
+    const steps: string[] = [];
+    class SomeComponent extends Component {
+      static template = xml`<div t-custom-plop.mouse="click" class="my-div"/>`;
+      click() {
+        steps.push("clicked");
+      }
+    }
+    const app = new App(SomeComponent, {
+      customDirectives: {
+        plop: (node, value, modifier) => {
+          node.setAttribute("t-on-click", value);
+          steps.push(modifier || "");
+        },
+      },
+    });
+    await app.mount(fixture);
+    expect(fixture.innerHTML).toBe(`<div class="my-div"></div>`);
+    fixture.querySelector("div")!.click();
+    expect(steps).toEqual(["mouse", "clicked"]);
+  });
+});