Support url template for dynamic url

Use lo-dash `_.template()` to handle url template with current
`test` object. The method support dot syntax to access nested
properties. E.g.:
```js
  lt.describe.whenCalledByUser(
  credentials,
  'PUT',
  '/api/users/${user.id}'
)
```

Signed-off-by: Clark Wang <clark.wangs@gmail.com>