Use `url_for` to generate Flask-Images URLs

Using `url_for` to generate Flask-Images URLs instead of a custom function. Use the route `images` (configurable with `IMAGES_NAME`).

e.g. `url_for('images', filename='image.png', width=30, mode='crop')`

I didn't removed the `resized_img_src` behaviour to maintain compatibility.

Close #5.