[FIX] qweb: support #{} syntax in attribute string interpolation

closes #113

tests/__snapshots__/qweb.test.ts.snap

@@ -93,6 +93,17 @@ exports[`attributes format expression 1`] = `
 }"
 `;
 
+exports[`attributes format expression, other format 1`] = `
+"function anonymous(context,extra
+) {
+    var h = this.utils.h;
+    var _1 = \`\${context['value'] + 37}\`;
+    let c2 = [], p2 = {key:2,attrs:{foo: _1}};
+    var vn2 = h('div', p2, c2);
+    return vn2;
+}"
+`;
+
 exports[`attributes format literal 1`] = `
 "function anonymous(context,extra
 ) {

tests/qweb.test.ts

@@ -511,6 +511,12 @@ describe("attributes", () => {
     expect(result).toBe(`<div foo="42"></div>`);
   });
 
+  test("format expression, other format", () => {
+    qweb.addTemplate("test", `<div t-attf-foo="#{value + 37}"/>`);
+    const result = renderToString(qweb, "test", { value: 5 });
+    expect(result).toBe(`<div foo="42"></div>`);
+  });
+
   test("format multiple", () => {
     qweb.addTemplate(
       "test",